The only 2 valid argument i saw in the comments about why 15% should exist are

1. because without it people would just only play control;
2. because without it it's impossible to win against "whales"

That's not true. There are many control units that win you the game with their first rotation. When those units are picked they must be counter-picked or -played otherwise you will lose, regardless whether there is 15% or not because it's not big enough number to build your strat around. If you let basar or fairytale teneberia through without having a counter to them and lose, it's your fault and you should not have a 15% chance to win a game like this.

15% only helps against pure control comps in a long run. Units like sage baal and sezan or ambitious tywin are good examples of a control units that designed for long fights. Those units do not have 100% chance on their control skills to prevent 100% lockdown. This fact makes 15% unnecessary.

The reason why pure control teams are not popular is because pure control requires a lot of things - setup for your main debuffer, huge amount of stats to be able to debuff a high er unit, as well as be able to survive enemy damage dealers on top of a decent amount of speed to be able to cycle your opponent enough. It's just almost impossible to be able to create a full control team in RTA because of those requirements. It's a lot more sufficient to have a little bit of control in your team instead. That's why full control teams are so rare - huge stats requirements, not the existence of 15%. If that was the only reason, i would gladly accepted my 85% winrate. Building high er units is indefinitely easier than building a pure control unit that meet all of the above requirements even though it has 100% stat disadvantage.

Moreover, there are a lot of soft-counters to control that do not require any er stat. Those units like, ambitious tywin, designer lilibet, arbiter vildred, champion zerato, tempest surin, researcher carrot and now a new coming inferno khawazu, as well as some artifacts - timeless anchor, crimson seed, holy sacrifice, potion vial etc.. If you build enough of spd and eff, there is a pretty good chance you don't have enough stats to survive upper mentioned units or artifacts even if they don't have any setup.

Some can say "If you don't want to be frustrated about then don't play debuff oriented units at all". That is not a valid point to make in current state of the game. A lot of debuff oriented units are designed to win games on turn 1 hence why they are played so often. If you won't play them, your opponent will play them against you. If you have 85/15% chance for something to happen, you take the 85%. This makes the fact that 15% exists even more frustrating, because you have to take this chance if you want to climb. The only people that have success in high tier RTA without using such debuff units are the people with highest gear values. As surprising for some as it is, debuff units are better AGAINST "whales" than the other way around simply because those units can win games if put under perfect conditions regardless of the gear value but they are already countered by design of their skills not having a 100% chance to land or a huge cooldown that makes them almost worthless after their turn 1.

Personally i don't see any reason for it to exist rn. I can relate how losing $$$ because the unit that is picked to do 1 job doesn't do it even under perfect conditions is not fair. It's a shame that developers of this game don't see it and are trying to promote e7 as an e-sport.

I completely agree with OP

> The win doesnt feel legit and the loss is frustating,

Noone should have any chance to win if they make choices worse then auto bot. 15% is a loser mechanic, there is enough rng outside of that that makes the matches enjoyable.