For me it's not just that tanking stances add something to gameplay in and of themselves. Rather, it's just that the balance for your outputs and needs as a tank would actually more heavily mandate certain skills to replace the stance's offset of that balance and thereby reduce the available choices within a certain situation. Having that simply overlying push towards the offensive or defensive allows for many more "shades" of combat choices in most scenarios, especially if not playing to exactly known circumstances. I feel like a similar system would have to be made available, and would likely require at least another button's worth of control, not to negatively impact the variety of play available to tanks upon removing stances. And at that point, there's no benefit in terms of combating button bloat. If the replacing system is more intuitive and engaging while providing mostly the same benefits, then by all means, replace the stances, but if not, then I don't much see the point.
Darkside I could take or lose. For me it's kind of like swapping from a manual transmission to a DCT, but less significant still — I'd probably miss the clutch (Darkside toggling) for a bit, but forget about it having been a thing soon enough, as long as I still have the same capabilities and decisions to make as before (e.g. if it didn't also come with automatic rev matching).
At present Blood Weapon is actually one of the reasons I like Grit more or less how it is, oddly enough. I have this very desireable offensive focus phase every time it comes up, or preferably lasting for a full minute (one BW to the end of the next, with 3 Scourge applications over that time), that I have to weigh my defensive capabilities against in the context of the fight. It kind of takes you for the ride and making you hope like hell you made sure to have enough safeties ready, in part because Grit is so costly to swap back to if you didn't. Sure, when I started the job, that seemed a silly underutilization of an integral and thematic skill, but the gameplay it caused eventually seemed integral and thematic itself. Just food for thought.
Also, absolutely agreed on Paladin. I don't think stance-restricted moves are the situation, either, with perhaps the slight exception of something like defensive vs. offensive variants of certain skills. Personally, I'd been tinkering around with a dual resource system (Zeal and Ardor) while actually making Sword and Shield oath each more usable in the opposite function as well (Shield as nuke damage and utility while Sword is increasingly capable of defensive action), but I've still got nothing concrete, sadly. So far the I've had more luck with revisions to Sword Oath itself as to produce a modified combo system that should improve PLD gameflow, making Shield Oath a bit more shield-based and similarly providing some more interesting, unique benefits, atop a few gameplay-affecting core traits (e.g. RNG mitigation being usable to turn enemy attacks onto each other, passive covering traits, and something "Sword and Board").