While likely true, the value of multiple UAs as opposed to a monoculture is that it increases the likelihood that a revolutionary implementation that _can_ scare well to large number of cores would be able to get any traction at all instead of just being stillborn because it doesn't duplicate all the bugs of the monoculture.

So my point wasn't that one would build such a UA by starting with Presto. My point was that there are distinct advantages to competition over everyone using the same rendering engine.