Almost everything you read about this is wrong... most of it put up by people who don’t know, who have never studied it, but have an opinion driven by how the heat of the moment influences their concern.
The Constitution empowers the Congress to impeach government officials.
The Constitution, quite wrongly today, assumes good faith and good judgement will constrain excess.
Presidents, once they are out of office, remain subject to that rule, and may be impeached, as they remain in offices of trust. They continue to be paid. They continue to be provided unique access to information. A security clearance is a foothold inside the process... but also a point of attachment to it ? As a former commander in chief, Presidents are not that different than other members of the military who, as officers, upon leaving service, remain as commissioned officers who are still subject to recall in the event of urgent need, while inactive. It’s not just a job.
The attendant risks are easily enough avoidable, though... if you choose to voluntarily relinquish those sorts of attachments. They can’t hold you to an obligation against your will... after you’ve already surrendered it.
However, once Trump is out of office, the whole point, from Democrats point of view... is that an impeachment provides a set of remedies that extends to denying him the right to ever fill public office, again. And, that’s why they’re doing it... hoping to prevent Trump running again in 2024.
But, they’d still need cooperation from Senate Republicans to make that happen... it requiring a separate vote on imposing that penalty in addition to voting to convict un the impeachment first. Odds of that happening, pretty close to zero... going negative and inverting over time, perhaps ?
Along the way, of course, they’ll hope that conducting a witch hunt impeachment, again, would provide them with some fodder to funnel to federal, state and local courts... in order to sustain their witch hunt in other venue in future.
That it’s obvious as a horribly bad idea isn’t a restraint on Democrats right now... apparent even just given a look at the range of horribly bad legislation already being tabled. They’re already seeking to conduct purges of political opposition throughout government, including converting the FBI and the DOJ into their own SS and Gestapo... while elevating Antifa/BLM to above the law, and declaring anyone who opposes them to be a terrorist:
https://freerepublic.com/focus/f-chat/3925167/posts
I will, somewhat, disagree with your take on this. As a retired Navy member I agreed, by written contract, to transfer to the Fleet Reserve. I elected to retire, PDJT did not elect to “retire”, that decision is baked into the constitution. The perks of a former president are those created by law and can be revoked by law as long as they are applied to all former presidents evenly, (bill of attainder and 14th amendment will be a problem if applied only to PDJT).