Even with the XP changes there will be stomps due to bad players and bad drafts.
Mostly only teams that change their playstyle from the early game or contain late game heroes make a comeback right now and it happens all the time.
XP lead is not the cause but just an effect for getting murdered the early game and the team that manages to get a lead kinda deserves to haveit because they played better and get rewarded for it.
Reward is making them a bit stronger and more likely to win the next teamfight unless the enemy changes their way of playing and if the team that is behind keeps making mistakes the lead will grow.
Most of the time the reason isnt XP but the bad early game play, enemy wont get an XP lead unless the other team has made considerable mistakes.
Edit: The reason for the snowballs is that to make a comrback you have to take forts or keeps or use the only true comeback mechanic of the game and get kills (its the only XP that gets scaled when youre behind) .
To take a fort or a keep means you have to fight for it when you are weaker so it basically makes the comeback possible only by fighting a talent tier or a couple of levels down.
Maybe the comeback XP mechanic should just be expanded to minions and scaled right, so soaking those minions could be a more viable, i know it is viable already but it still takes more time than to brawl and in the games we are on a timer for the objective.
Edit 2: TL;DR if you want comebacks and the only way to do so right now is by fighting when you are weaker, instead of making them equally as strong and making early game not matter you should give other ways to make a comeback like soaking correctly and things like that.
So if the team thats ahead and the team that is behing both soak as much the team behind will be able to catch up faster and to stop them from catching up the team that is ahead has to actively stop the enemy from doing it instead of just not dying and doing whatever.
This way teams ahead keep their lead by keeping up their good play actively instead of just rolling with it.
Sorry for writing an essay.