Do we know for certain that Umaril was the Last Ayleid?
He was the last of the last of the last of the Ayleid Sorcerer Kings, and the last "cultural/ideological" Ayleid.
Many Ayleid bloodlines probably exist to this day in Tamriel, having long fled the Heartland and assimilated into various other cultures. There are probably many Bretons, Altmer, Bosmer, and even Dunmer around with Ayleid blood, and there may be scattered tribes of "wild elves" operating on the fringes of society.
But there's more than blood to a nationality. There are customs, social norms, art, language, religion, architecture etc. The Ayleid culture no longer exists, although bits and pieces were borrowed for the Empire of Man.
In that regard, Umaril was the very last Ayleid. Not only that, but one of the Sorcerer Kings. Their champion in fact. Something more and at the same time perhaps less than other Ayleids, this one was. Half Ayleid, half Divine, and infused with Daedric life force and gods know how many countless souls of human slaves through his obscene necromantic sorceries. He was the last carrier of the Ayleid Ideology, that of Elvish racial superiority, of enslavement of humans, of the particular Ayleid religions, which seemed to combine Daedric and Aedric influences. In that regard the Ayleids died with him at long last, although for all intents and purposes that culture died out a long time before he made his brief return.
It's interesting to note the parallel between the Dwemer, the Ayleids, the Falmer, and other "lost races of Elves." All or most of these races occupy a similar cultural/mythical space in their respective continents/cultures. The Dwemer and the Ayleids seemed to both practice perverse or wicked sorceries with dubious goals of apotheosis, and both are now vanished although through different means. Both also occupied a Tower. I wouldn't be surprised if the full true story of the Falmer is delved into and they perhaps had a similar fate, if indeed the next game is set in Skyrim. I'd expect to see Falmler ruins, Falmer artifacts and perhaps even a "last living Falmer", if the trend continues. Perhaps those similarities are intentional and serve an illustration about cycles and myth-echoes in the Elder Scrolls.