In the case of Poland you did have a society on the verge of collapse. There had been a deep recession starting in 1978. Until then decade had seen massive hard currency loans supporting rapidly rising living standards and white elephant projects. Eventually the credit dried up, the Party sought to stabilize things buy large price hikes combined with frozen salaries and presto - massive labour unrest, which in the context of an authoritarian society automatically meant political unrest. The result was even more economic disruption culminating in a crackdown. The rest of the eighties saw very poor living standards (you really couldn't get anything in the stores on any sort of regular basis except for dairy products, bread, eggs, and root vegetables - not meat, not shoes, not toilet paper, not soap, not anything). The economy was kept from full collapse by the Soviets reacting to 1980-81 with signfificant subsidies starting in 1982 and increased private transfers from the West coupled with an increase in the hard currency store system (with change given in scrip).

In 1988 even this wasn't working as a decade of zero capital investment caught up with things. The Party asked the elder brothers for a sharp increase in subsidies and were turned down since the Soviets had financial issues of their own. That meant that living standards had to drop again. Given that the Party had zero popular legitimacy at that point, this left two choices. Either get the legitimacy from the opposition (unlike elsewhere in the Bloc you had a large well organized mass opposition movement operating underground - the legacy of 1980-81) or to resort to large scale bloodshed (and the unrest would have made the economic crisis even worse).

Moscow did not want its new and improved image in the West getting destroyed, so in late 1988 it greenlighted formal high level negotiations on a power sharing agreement that led to the legalization of trade unions, an opposition press, and most importantly semi-free elections in June 1989. One third of the decisive lower house was up for grabs, and all of a new, weaker upper chamber. The expectation was that you'd get a national unity government with Solidarity as junior partners.

Except it turned out that the Party was even more unpopular than anyone realized, even the Solidarity leadership itself, and the Party and its allied 'independent' candidates lost all except one of the seats that were open. To make matters worse, the gimmick of allowing several Party candidates to contest the closed seats turned out to have real consequences as voters systematically crossed whichever candidate was higher up in the Party hierarchy, meaning that the Party leadership was mostly unrepresented in the suddenly important parliament. Solidarity now had veto power over the choice of President (elected by the combined chambers and quite powerful) and used that to force a Solidarity led government with only the 'power ministries' in Party hands, plus an agreement that the next elections would be fully free. By the end of the summer the Party apparatus was disintegrating and the secret police was burning mountains of documents in anticipation of dissolution. The regime was dead and the only way it could have been revived was through a Soviet invasion with no guarantees about what the Polish Army would do.

Hmmh, somewhat of a long comment, sorry about that.