I wouldn’t go that far. First, throwing someone’s livelihood out the window for something like this is a bit…brutal. Especially the person who suggested it, rather than the person who decided it was a good idea and pivoted to it.
Second, I doubt any loss from it could be possibly quantifiable. There may have been some players that quit playing specifically due to RSV being removed, but I doubt it was many. There probably were a rather larger number where the removal of it either directly (ie. they consciously thought about it) or indirectly (failure to find a spec they enjoyed, where RSV would have filled that niche for them) contributed to them quitting playing. Even so, it’s almost certainly impossible to determine how much of that can be attributed direct to RSV’s removal, and what the actual monetary loss from it was, or even be able to ballpark it at the coarse order of magnitudes level.
Have to keep in mind how revenue works for Blizzard. Even assuming 100% of lost subscription money is lost net (which it isn’t, the fewer average players, the less money they need to spend on server maintenance, bandwidth, CS staffing, etc), and assuming everyone is paying with the 15/mo sub (which actually may be a reasonable estimate, since the additional income from subs paid via WoW tokens (which cost $20 per month of sub time) probably largely balances with the lost monthly income from higher-duration sub packages), a million dollars over 4 years would require about 3000 people to have quit day one of Legion, or proportionately more of they quit partway through that period. Amplify that even further for your plural “millions”.
Still, the core sentiment is likely accurate. I suspect they did lose some players directly over RSV’s removal (just as they almost certainly lost players over the revamps to MM, Shadow, Demo, Aff, Enh, etc), and I also suspect that MSV’s relative unpopularity compared to RSV (ie. substantially fewer players finding it to be an entertaining spec they were interested in playing) probably contributed to the pile of reasons a number of other players quit playing.
Then again, designing classes with the goal of maximizing revenue is not a road we want Blizzard going down (or rather, going down more, since they are clearly already well along that path, both in class design and in greater game design). They should be designing classes with the goal of maximizing player enjoyment. MSV definitely failed that test.