I can't speak for "under any circumstances" but I suspect the vast majority of tax payers already outsource their tax computations to either one of the major software vendors, one of the major tax services or their own financial institutions / private accountant. I assume this because otherwise I would not expect "IRS makes a self service portal" to be headline news or a game of politics.

Beyond that though, the thread discussion was about whether there are any mechanisms by which the government pre-filling people's taxes could '[make] "tax increases by stealth" easier'. I proposed there were, though with no claims to the degree to which that actually had real world impact on tax payers and their behavior.

If we assume as you say that people "want to pay what they have to pay and get on with the rest of their lives", then it seems reasonable to think that anything which reduces the amount of effort and thought necessary for the process also has potential to '[make] "tax increases by stealth" easier', simply by the fact that it reduces the amount of thought one needs to afford to the concept of taxes in the the first place.

The alternative would be to propose some mechanism by which the government pre-filing people's taxes would increase the degree to which they pay attention to their taxes. Or at a minimum one needs to argue that people would pay exactly as much attention as they do now and thus it would be a net-zero change in focus on taxes, but if that's the case, then any compliance increase would be a function of making it easier to be in compliance at all, which is tangental to whether or not it '[made] "tax increases by stealth" easier'