Hands down, the most beneficial Mosaic perk prior to COVID induced flexibility, was free changes.
However, as a Mosaic member myself, I've taken advantage of other significant perks that remain exclusive to Mosaics.
For me, the most lucrative benefit is the ability to use points to upgrade to even more space, and at a fraction of the value (calculating the $$ to points conversion)
For reference : on a most recent flight, the upcharge to Even More Space ran around the $80's, while the points redemption for that same seat upgrade was 600 points. 1:1 conversion makes that $6! Beyond generous imo.
Also, given the low number of elites, I almost always wound up with the seat next to me unoccupied in even more space section.
I've frequently been the only passenger pre-boarding in Mosaic line. Hence, the value of certain benefits is inflated by virtue of the low number of elites.
The exclusive Mosaic phone line should not be underestimated either.
Granted, value derived will depend on future travel patterns, something that remains uncertain for the majority of the traveling public; but $75 could be the value extracted of just one leg of a trip, leveraging the seat upgrade possibility and the 2 free bags alone.
Just my take.