The quality of the update is really the issue here, not that they are mandatory. The fact it is mandatory is no different than MMO patches being mandatory. MSFS is not a single player, disconnected game, even if you turn all the online features off. If you wish to participate in its use then you need to be running the right client version.
If the patches were 100% perfect all of the time, with zero bugs, and took 5 minutes to download, and install then nobody would be complaining about mandatory patches.