Bi-Levels are often used because making long trains means you need really long station platforms. We already have very long station platforms.
However, for every GO platform, 2 new platforms need to be built.
In addition, we may end up choosing trainset that benefits from a platform raise.
Plus, a lot of the infill stations may not have room for full-length trains.
Then shorter level-boarding platforms may still provide a faster boarding experience with an efficient bilevel, to the point that it feels more like boarding a subway train than a GO train. It's not the era of waiting in lines and old-fashioned paper tickets anymore, a farecard tap and a level walk onto an European bilevel, feels very much like stepping into an up-sized subway train in many ways. Also, the London Overground is more corridor constrained than Metrolinxs' GO corridors are, with trains needing to fit under bridges and occasional small sections of tunnel. Metrolinx does not have such a restriction.
I agree single levels are a great choice, but it is far from the only choice because of Metrolinx's network is already optimized to fit large bilevel trains. Because of infrastructural reasons, it is possible that bilevel EMUs may become a better fit for SmartTrack, and given a specific price actually move more people than a single-level. But changes are being made to the point where it may be possible to run subway-like 2-minute headways once the underground Metrolinx corridor gets built (various proposals mentioned in the "Metrolinx 2031" plan). At which point, it begins to heavily favour single-level subway style trains as nothing in the world outperforms a well-tuned subway train route in train frequency. But that may not happen to Metrolinx corridor (2-minute headwdays) for a few decades, well beyond the lifetime of a bi-level EMU.
There was a reason we ended up with long bi-levels pushing an incredible crush load of 4000 people -- one of the most passenger-dense passenger trains in the developed Western world (with no people hanging off the sides of trains, like in developing countries). We were train-trip-limited -- One upon a time, we had to run on freight corridor, and it cost quite a bit per train run in the day, so we had to cram as many people as possible per train. The invention of the Bombardier BiLevel in 1979 dramatically improved the efficiency of GO, and it remains popular to this date, and will probably be part of the GO network till the 2030s or 2040s (electric locomotive driven, for the limited expresses). Now Metrolinx owns the majority of the GO network, this is about to change -- possibly goodbye bilevels within a few decades. So it could happen. But the Metrolinx GO network is at a point where SmartTrack may actually end up being the most efficiently served by bilevel EMUs (platforms, platform length, number of commuters, etc).
Even with SmartTrack infills, the station density is still MUCH LESS than a typical subway, and the performance profile of a bilevel EMU may actually be a good fit. It could happen either way. But for tight train slotting between UPX trains, it's quite possible they'll use single-level EMUs, in a pursuit towards eventual subway-tight headways along the SmartTrack route (improve this, improve that, resignal, then finally the USRC bypass tunnel, remove discontinued crossovers, and bam -- full physical separation from all other GO routes -- 2-minute headways become possible on the GO network! But that's likely not till after the 2030s, and possibly long enough beyond the lifetime of a bilevel EMU.
