The Democrats are keeping Biden away from Trump because they know as well as you do that any debate would be an absolute disaster, but probably not for the reasons many people are focusing on. Due to the fact that Biden is only fit for a retirement home, protecting Biden's reputation isn't exactly the reason they're not debating. 

They're not debating because it would allow Trump to give his side of the story about various narratives and debate Biden on them directly. With Biden wholly unable to fight back with any reliability, the narratives would be torn to shreds in front of millions of people. 

As Jonathan Tobin wrote at The Federalist, the real "debate" will occur in courtrooms where Trump will have to fend off accusations of illegal wrongdoing, draining his coffers and distracting him from the campaign trail: 

The heavy lifting for the case that Biden shouldn’t deign to debate Trump won’t be done by his campaign. It will be the work of Democrat prosecutors in trials like the farce being played in Manhattan, where Trump is being tried for a campaign finance violation by a state court rather than a federal one because of the absurd claim that paying hush money to Stormy Daniels was an unreported contribution. And if jurors in deep-blue New York City convict him on this ridiculous charge, it will be all the reason Biden needs to declare that he can’t debate a felon, even if everyone involved knows any conviction won’t withstand appellate scrutiny. The other Trump trials, if any or all of them are held before the election, will provide the same service to Democrats.

Democrats will have their debate, it just won't look like ones you've watched for decades. It will be incredibly one-sided and the only response Trump will truly have is being proven innocent. Not that this will make any difference to Democrats or the corporate media. 

Trump is guilty of being a Republican and, of course, guilty of being Trump. 

The guilt of being Trump is what they want to maintain, and that means protecting some of the aforementioned narratives. Chief among these is the January 6 narrative. While any news organization worth their salt would know that Trump wasn't to blame for what happened at the Capitol, giving Trump the ability to talk about January 6 in the open would have disastrous results on this narrative. 

It should be well understood that a great many people still believe the media narrative that Trump tried to lead some sort of revolt. A CBS poll in January showed that respondents claim they "disapprove" of the actions of those who forced their way into the capitol. This allows the Democrats to sell the idea that a Trump presidency will be dangerous, evil, and detrimental to anyone who doesn't fit in with conservative ideals. 

So long as that remains in play, Democrats can continue to play on people's fears. Having Trump in court, no matter the reason, helps sell the idea that Trump is a criminal and not fit for office. 

It's a dirty tactic but this is what the Democrats are left with. They have no hope in Biden. They have nothing to stand on in terms of victories and successes. All they can do now is lean as heavily as they can into assassinating Trump's character. All they have to do is protect the narratives they've crafted around him. 

But I'm not sure that's going to be enough.