What is the timeframe for "eventually"? The Series I BiLevels have been around since the late 1970s and underwent their most recent refurbishments a few years ago. A coach is basically a shell and is much easier to keep going for decades than a locomotive or transit vehicle.
As for what's the point... eliminating diesel emissions is not enough? Why replace perfectly functional rolling stock that you can get decades of use out of? If you want to purchase EMUs for service expansions, by all means, but making what we have last as long as possible in addition to buying new equipment would be the best decision.
Which is why I don't expect Metrolinx to do it.