The institutions did their jobs. Trump failed to seize power extralegally, despite populist support. The US system of government could be better, but it is functional.
There is neither a Constitutional nor a rational basis for a special unique exemption for past Presidents, and fortunately state and federal prosecutors seem to recognize that.
Sure, but using criminal process or any other power of office to punish partisan enemies is a problem whether or not the target is a past leader; there's no special danger around past leaders. if anything, there is less danger around past or current high government officials in general, and past Presidents in particular, because of the extraordinary degree to which those persons tend to be visible and well-connected politically.
Indictments on 91 felony charges across three different sovereignties in four different proceedings is...not what most people would describe as zero consequences.
(Also, there are several other active state criminal investigations relating to the fraudulent electors scheme, some of which have produced criminal charges against other people and any of which could very easily also result in further indictments for Trump.)
While I believe in due process, justice delayed is not justice.
And the body that should have sanctioned him (the elected government) voted down any punishment.