It's not even a valid debate when a party on the ballot in all 50 states isn't even allowed on stage to share their perspective and views.
Should be criminal.
The bar is 5% of votes in the previous election, or polling at 15+%, isn't it? That doesn't seem unreasonable.
If you changed the criteria to just being on the ballot in 50 states, the Green party would definitely push to get onto the last handful of ballots (they were on 45 of 51 in 2016), so that's 4 people up there. I would think that the Lincoln Project would have been pushing hard to have someone on there as well, and would probably get the required signatures for each state without too much trouble if they started early enough (and made being on the debate stage a big part of their push for getting onto the ballot).
A 4 or 5 person debate wouldn't be an upgrade over what we have right now, it would be a mess (especially since the moderator would, for good reason, give most of the time to the Dem and Rep candidates, which would upset the other candidates). It'd be nice to have more 3rd party recognition/representation, but it'd have to start with them getting more votes (which is all the more reason to push for things like Ranked-Choice voting, so you can vote for a 3rd party without it just being a waste)