This is worded such so that if you set it on fire at a creature feet it can move out of it, because the correct way to do that is to hit the creature in which case they take 5 more fire damage from the next fire attack. It's an area denial tool, not a "I win" tool. Fire surfaces right now are "I win".
Funniest thing is, that, of all the systems, it's 5e they try to make those surface shenanigans work with, where lingering effects have ben severely toned down from previous versions... if they added stuff like that to, say, 3.5, it wouldn't have been nearly as big of a problem.
There are a lot of lingering locational effects in D&D 5e. And it's fairly easy to work in alchemical versions. Alchemist's fire is already in. The differences between grenades and spells are usually DC and radius. And around about 6th level, spells start seriously outpacing alchemical grenades.
Yes, but especially at low levels, those effects are not long lasting and easily overcome, they usually have only minor impact on battles.
