I think the "Champions League style" set up VILLA MOLE suggested would give those teams a mixture of games against teams at their own level while still offering them the chance to play the bigger teams (and, crucially, still get money from hosting said opponents).
The money thing is important, but I think the objection to doing that sort of format is that the matches you'd play aren't then massively different to tournament qualification.
There's already plenty of opportunity to play eight-to-ten matches where you're divided into a group ranked by rough ability from best to worst, so why do the same thing again.
You'd be playing different opponents rather than the same team home and away, so that provides more variety.
Top teams can aim to qualify for the Nations League Finals, expand the playoffs for World Cup Qualifying so even mid-ranking teams have a chance of having something to play for in the final games, so you don't get many dead rubbers.
At the moment there are too many meaningless games at the end of qualifying. There were very few at the end of the Champions League Group Stage last season.
