So, that's the situation. One player in particular was quite upset by these spells, and declared them to be 'design failures' for meaning that player characters would be put out of the game, leaving the players nothing to do. This was specifically worse than hit point loss, he stated; I believe the argument was that hit point loss could be responded to by, e.g. healing or resurrection magic, whereas the Prismatic Spray in particular had no chance of being undone in combat. I had numerous counter-arguments, which I won't get into here; what I'm interested in is hearing the thoughts of others. Do you agree with the players?
He's wrong. Prismatic Spray can be countered by Counterspell or Indomitable or Lucky or proactively by Anti-magic Field (as he indeed attempted). It can even be "Undone" by a Gate spell from another PC, or by the targeted PC Plane Shifting + Teleporting back into combat, or by everyone else Plane Shifting
out temporarily to rejoin the targeted PC. (BTW, you could get the same banishing effect from Banishment, as well as Prismatic Spray.)
Furthermore, his design goal is incoherent. Putting a player "out of the game" for a few minutes of game time isn't a design failure in the first place, and if it were it could still happen with HP loss anyway--there's no guarantee anyone will resurrect you anyway. (Revifify has a short range, and your Revivify caster may be busy with other things during combat. Furthermore, consider PCs who are e.g. swallowed by the Tarrasque.)
He's just wrong.
However, if it were done when 'tis done, then 'twere well it were done quickly. If you're going to squish the PCs like bugs, it shouldn't take multiple game sessions to do so. (Although who knows, this may be his own fault for taking up too much time with "numerous" arguments and counter-arguments.) I could buy the argument that it's a bit of a design oops on the DM's part that the combat lasted longer than one game session, putting the player's (or players'?) desire to attend the next session at risk, since he might very well spend the whole time sitting around spectating if the combat runs even longer. I doubt you
intended for the inevitable loss on the players' part to take more than one session--from the previous thread I thought they were going to lose quite quickly, at the rate of about 1-2 PCs per round (depending on whether Vlaakith was done with AMF or not).