So, 5e is a weird thing, where the players can be engaging in a gamist way, but the GM is fully in HCS sim. This is because the combats are so available to the GM putting their thumb on the scale, both in design and in play. There's no way for the players to ever know if the GM just didn't push the planned second wave to compensate for their bad rolls or a bad tactic, or vice versa -- no way to know if the second wave was always planned and not a reaction to the quick dispatch of the first.
And then there's the thing that some players might want the feel of the challenge, but not actually challenge. 5e does this very well, where failure is rarely actually on the line, but the concern it could come is often present.
Now, none of this is to suggest that a table can't lean hard into 5e gamism and drive it that way. You can, but the system isn't super helpful here because so much of it requires GM rulings to even operate.