The Duffers have said from the start that they know the ending, and are just working toward that (I take your point about padding - I expect they originally envisaged doing the entire story over 3 or perhaps 4 seasons, but s1 was such a hit they got pressured to extend it; it's good that they insist s5 will be the last - although there's talk of a spin-off
WRT Max, Vecna needed 4 kills to create the gates necessary to push through into the real world. He'd likely have had other victims potentially lined up, but Max was already 'prepped' and put herself in that vulnerable position to give him the easy target.
They're my theories at least.