A shotgun clause is probably the best method out of a bad lot, but it still has the potential for abuse.
If a partner is going through financial difficulty (outside of the company), then the partner with a better financial position can force a sale at a depressed valuation.
Say my mother has a massive stroke and requires expensive ongoing medical care. After a while, I'm financially drained. At that point, my partner can trigger the shotgun clause at an amount larger than my ability to scrape up cash, and wham I'm out. Out with a small payout, but out regardless.
There isn't a silver bullet to these kinds of issues. At the end of the day, people are involved, and people are complicated and difficult.
I agree with your premise, reasoning, and conclusion. However, I'd disagree about what your biggest fear should be.
A horrible, Fundable.com meltdown should be, by far, a bigger fear than getting abused by a shotgun clause.
Even in the land of the do-over, I can't see how this won't destroy the confidence that investors would have in either party going forward. Your future is so very much more important than your ownership in a startup with a co-founder who has no qualms about ripping you off.
As you say, there is no legal solution to the problem of bad humans. But there's no legal solution to death or car accidents, either; what we have are legal measures to manage those risks, in the form of contracts of some kind.
A shotgun clause is a last-resort clause, and from its very nature, it appears to me that it would only be invoked when you would most want it to be invoked.
( disclaimer: mr_luc has no personal experience with shotgun clauses and may very well be talking out of a suboptimal orifice. )
In theory given enough time you could go to investors and raise cash (binding in the event that your bid wins) backing a bid. It's the job of investors to allocate capital, after all.
Of course, I realize that theory does not always match reality.
