Early game only? How long does it take to finish one item? 12-15 minutes?
Ignite basically ensures dominance for that time without any great outside influence.
And you do not find it an issue that many assassins/burst mages have enough damage in their kits to kill a not tanky opponent already then they stack external damage boosters like duskblade/luden ontop along with ignite to ensure that even if a portion of that initial damage is indeed blocked by barrier the target still dies?
Like i have seen mages blow up from a fizz at 6 with sheen only, and the fizz'es missed their ults, and fizz is by no means one of the strongest ones right now.
As for botlane, right now Lucian, sivir, MF, vayne, draven , xayah and new ez? Are all in the top 10 of marksmen, all of them are strong from a early point and several are extremely burst heavy.
Supports like leo can legitimately chunk like 40-60% of a marksman health in a single rotation at 6 with a couple of auto´s or so, add ignite doing 205 outa 875-950 max health and suddenly its nearly possible for the supports to completely murder the marksmen on their own.
Its far too powerful in comparison to the other options, this is easily seen by how damage hasnt decreased at all while exhaust still isnt picked.