I guess if you stuck compliant tyres (sub 33mm without spikes or studs) and Drops (and therefore probably brake/gear levers too) on a disc equipped 29er MTB then it would be compliant as a crosser, but then it wouldn’t really be an MTB anymore.
Maybe the differentiation for MTBs should be tyre width 34mm or more (spikes and studs allowed) and Drop bars not allowed, that, to my mind is enough differentiation to keep the two separate, it would be a significant enough amount of faff to make a normal 29er MTB, compliant as a crosser and vice versa that it wouldn’t really be worth doing and thus the disciplines remain distinct and separate, with enough migration of common/similar equipment that competitors in one might take an interest in the other (hardly a bad thing in my view)….
As for discs appearing on road bikes, I suppose the only real resistance you might come across is riders/team not being keen on the additional weight of a rotor (of whatever size and material) at each wheel hub, you’d need to loose a lot of rim weight to offset that particular issue (plus of course tubs will now stay glued), it can be done, but I can see some potential for a little bit of resistance…