Slightly different situation.
Yes, 4-man RM is intentionally limiting outside of the intended design. However, where possible, the game should still be designed to allow that. There was no reason to change Spear to DPS gain except to satisfy DPS-centric players. Now I'm sure you'll say "There was no reason to not change it just to satisfy undersize-centric players." That's true, but also remember that changing it literally makes some undersized content impossible. What increasing DPS does is it speeds up kill times by a minor amount. So the tradeoff of removing content (even if it's derived from player-imposed limitation) isn't worth it.
Also, I'm not following what you're saying about the Tank Swap mechanics. They don't need to change current content to enable undersized runs. In fact, a lot of "forced" tank swaps are actually avoidable anyways. You can just carefully manage aggro then Shirk to a DPS for one tank buster to drop the stacks, then Provoke the boss back. I suppose the difference I'm seeing is between creating a challenge that doesn't currently exist, versus deleting one that already does (and may be quite beloved to some players) in exchange for something that is not similarly enabling, like a DPS increase. If they replaced the old Spear effect with a new one that enabled certain challenge runs in its own way, it wouldn't be quite so bad.
However, well outside of the issue of self-imposed limitations, Spear did still serve a valuable purpose. I found it not just useful, but even necessary to use at certain points when doing 8-man content with parties of mechanically weak players. And for those situations, your argument for irrelevance based on self-imposed limitation holds no weight.