I agree that Trump would have very little chance of becoming President if he was jailed before 2024 and kept in jail. I was imagining a “jailed at the last minute” scenario in which Trump’s credibility as the opposition leader imprisoned by a President he accused of being elected illegitimately could have been enough. I presume that before Trump became the front-runner Biden hoped that jailing a former President wouldn’t be necessary to stop him, and after he became the front-runner it was probably too late.

Even jailing Trump early wouldn’t have been risk-free. Republican Congressmen would publicly denounce the act as illegitimate, impeach Biden, and try to pass a law to free Trump. The impeachment and the law would go nowhere, but the controversy (and the likely riots) could sink the Democrats in 2024 against whoever the Republicans did nominate. Then that Republican would be inclined to have Biden prosecuted, which would be bad for more than Biden himself.

(And trying to jail Trump but failing for whatever reason would probably sink the Democrats against Trump.)