Bear with me on this, area denial is super important in this game, as its the best and sometimes only way to clear out deeply entrenched enemies, but it also requires the least amount of risk to be effective. Let’s look at the mercs that can instagib the easiest and how little they have to commit to completely removing an enemy from play:
A Fragger who cooks his grenades behind cover is barely in danger, and there’s little an opponent can do once nade is flying towards their face. The only real counter to this is to guess when he’s cooking the grenade and charge him.
Skyhammer gets to “set and forget” his airstrike, exposing himself for only a moment. The delay however lets you escape if you’re aware enough.
Vasilli gets instagibs on his headshots. Granted this takes a reasonable level of skill to pull off, but there is virtually no counter play to being caught in his line of sight. If you don’t know where he is and he catches you first, game over, see you next wave. Vasillis who can land jump headshots from cover are literally risking nothing for the ultimate reward.
What’s the point of toning down low-risk/high-reward gameplay? It encourages players to play aggressively to secure their kills. It makes good aim and movement the most reliable means of downing an enemy for good. It also increases the likelihood of revives, which should lengthen fights and keep the tempo of the match higher. If you wanna kill someone for good, take out your primary and shove it down the other guys throat. You can even teabag afterwards!
Area denial characters already accomplish their role by simply damaging enemies in an AOE, essentially telling the opponents “leave or die”. They don’t need instagibs to do this.
Is this a buff to medics? Absolutely, but having support characters being the most vital team member in the meta (TF2) is a good thing. It places an emphasis on team-work, where both protecting and killing the medic becomes even more important.
TL:DR Removing instagibs forces players to use their primaries more, and more frequent revives lets players fight for longer.
-instagib = +skill ceiling & +fun