Socrates is side-stepping the issue, as he always did, taking advantage of his friend's inability to articulate himself well.
One can see from his friend's meandering talk that he was trying to describe how excessive love of gain can corrupt, and later how evil men would deceive others so as to make gains by things which the violated party will later realize (or perhaps not realize) are worthless. Think snake oil merchants.
However, Socrates, in his usual style, takes advantage of imprecise language to twist meanings around until he's altered the topic of discussion (as his friend suspected, but couldn't articulate) into something where he can be demonstrated to have the right answer (all men love gain) with a notable absence of discussion about his friend's concern about excessive love of gain and dishonest gain.
There's a reason why they killed him: nobody likes a smart ass.
Where you look for trickery you will find trickery. Look for the wisdom and you will find that instead.
What you actually mean by "excessive love of gain" is that some people either do not see or do not count the externalities of their activities - they see only the gain for themselves and not the loss for others, for example.
For example, a thief doesn't care that you lose $1000 for every $100 he makes, so he is happy to go about this wholesale destruction of wealth for his own gain. But then he is not evil because he loves gain, but because he disregards other people's losses.
Taking a less evil example, a man buying a cheap pair of shoes from walmart does so for the gain of what he perceives to be a good-value-for-money pair of shoes. He either does not care, or more likely does not realise that these cheap shoes will probably fall apart soon (thus costing him more to buy a new pair of shoes) and that some people on the other side of the world were exploited to make those shoes, and that this caused them suffering, and could be a direct cost to him too in this inter-connected world, both in opportunity cost of some perhaps smart people being exploited to make shoes, and in the losses caused by a possible war somewhere else. This man is also not evil because of his love of gain, but simply because of his ignorance, or perhaps stupidity.
All men do love gain. And, as Socrates argues, all men should love gain. But some men are evil (unrelated to their very healthy and natural love of gain), and some men are fools (so they do not know how to pick things which will provide them actual gains).
The classic "loves gain too much" example of the last couple of years, investment bankers, might fall into either category. But whichever the case, they are not evil because of their natural love of gain, but because of either their short-sightedness ("Prices will keep rising forever!"), or their lack of caring for the externalities of their actions ("I don't care if the economy loses $100m so long as I get a $1m bonus").
Gain is good.
Greed is good.
Stupidity, ignorance and lack of compassion are evil.
tl;dr; :: The point that Socrates successfully makes is that it's not love of gain (excessive or not) which is the problem, but the fundamental nature of the person expressing that love. Everyone loves gain, but some people don't give a shit who they hurt to get it.
While the final, deplorable behavior is disregard for the social cost of ones own gain, one cannot discount the germination and growth of greed as an otherwise upright man is slowly corrupted by the gains he makes, and his priorities shift towards higher gain as concern for the damage done to others falls by the wayside. The quote "for the love of money is the root of all kinds of evil" did not arise from a vacuum, nor did "absolute power corrupts absolutely".
Ultimately, one can argue that it is a weakness of character, but thus far it's been incredibly rare to see those with great power (through gains) remain uncorrupted for long.
But could you not argue it from the other perspective?
In your first example one could say it is your thief's love of gain that causes him to disregard other people's property. Similarly, with your shoe example, it is the man's love of gain (shoes for cheap) that allows him to willfully disregard the exploitation involved in the construction of the shoes. This man's desire to get a good deal on shoes (gain) prevents him from purchasing more expensive locally made shoes which were not made through the exploitation of some laborer in a developing country.
Thus it is the love of gain that causes people to disregard property, the suffering of others, and act irresponsibly with other's money (following from your banker's short sightedness). That, to me, implies that it is the love of gain that is the root of many evil actions. There are many good stupid people in this world.
Traditionally, "Greed" has meant desire for gain out of balance with external costs.
I mention it not to pick a nit, but because I often seen the "greed is good" idea justified in the same way you (rightfully, in my opinion) justify "Gain is good".
Greed, by definition, means the gain which is no longer good!
> He either does not care, or more likely does not realise that these cheap shoes will probably fall apart soon
Or, he's got enough money for a cheap pair of shoes and something else he needs, but can't afford the latter if he pays more for the shoes.
> Taking a less evil example, a man buying a cheap pair of shoes from walmart
BTW - I've done the relevant experiments (including shoes). Yes, some of the walmart stuff doesn't last as long, but the price discount more than makes up for the difference. (In other words, something that last 2x as long costs 3x as much.)
Since you disagree, what products, specifically, support your assumption? (You have relevant experience, right?)
One can see from his friend's meandering talk that he was trying to describe how excessive love of gain can corrupt, and later how evil men would deceive others so as to make gains by things which the violated party will later realize (or perhaps not realize) are worthless. Think snake oil merchants.
However, Socrates, in his usual style, takes advantage of imprecise language to twist meanings around until he's altered the topic of discussion (as his friend suspected, but couldn't articulate) into something where he can be demonstrated to have the right answer (all men love gain) with a notable absence of discussion about his friend's concern about excessive love of gain and dishonest gain.
There's a reason why they killed him: nobody likes a smart ass.