Symbolically though, if government ran a surplus for x years during a particular ten year period, that would be sufficient to say "look, we've mostly balanced the books, we have been running surpluses during some years".

Again it's about public perception more than anything else. When national debt is climbing towards heights only previously reached during WW2 and government has shown zero ability to run a surplus in over a decade (I think Clinton ran surpluses?) then people will simply not trust politicians with the authority to borrow the necessary amount for Keynesianism to work.

That's it. The rest is just noise. It's just too easy to spend other people's money, under the guise of 'public interest'. Even if the politician is not corrupt, and not, say, channeling public funds to friends, outside of a few obvious cases (foreign invasion, giant earthquake, meteor strike, e.g.) it is difficult to hold politicians accountable for, say "good intentions gone awry".

The United States did quite well for a half century or so (only dipping into debt to pay for wars). There was the moral precept of 'no taxation without representation' and the early government understood that debt incurred the future taxation of people who would not be able to go back in time and vote against the spending[0]. That's all been abandoned long since.

[0] it is often said that "governments are not like families" and so that debt dynamics are different. Well, that is indeed the case. At least when individuals enter a debt, there is a contract of understanding between a single person and the lendee which means that the responsibility for repayment is on the borrower and the consequences of default are on the lender (except in a few morally questionable cases, such as academic debt). When governments enter debt, the payment is the responsibility of, potentially, someone else (there's the 'spending other people's money' again, except this time, at a societal level).