I suspect such an extension (partly) hinges on the construction of the terminus at Finch station, YNSE might generate too much chaos for an LRT terminus to be dug up and built at the same time, but this assumes this extension would even get approved by that time (lol).
Or maybe not. Apart from maybe having to worry about a small bridge and an underpass, this 6km~ extension built at-grade seems very feasible. Finch West has made an excellent example of how (relatively) cheap and fast at-grade LRT construction can be built. Though I wouldn't be surprised if the proposed budget for an extension would still approach 1 billion because of Finch station.
I'm praying Finch West opens on time (and before Eglinton's new scheduled opening for greater effect) so there'll be a lot more support for the construction of more lines like it.