I'll probably wait to dive into non-GO-related Expo 2025 discussion until the bid happens...
Separately of "is it a good idea?" given Ontario finances, but it is a completely different question and potentially different answer than "will it help GO infrastructure?" which was the original intent of my mentioning the expo. Since this is not a done deal and it may all be moot, the last thing I will say on the ranking matter ("vs Olympics" "vs PanAm"), is generally for Canada itself, expos are ranked above Canadian Olympics (which are ranked above Canadian PanAm) in infrastructure spinoffs, given Montreal Expo 67 and Vancouver Expo 86 experiences (The 1976 Olympics is a different story altogether).
Just like '67 and '86 started the respective cities' first subway/elevated systems -- given Toronto's fully schizophrenic history in public transit, expos provide a form of transit project encouragement and anti-cancellation policy for Canada's first electric RER system (European-style migration of traditional rail commuter service into a high-frequency 2-way transit integrated system now fully worthy of being part of subway maps) -- a GO transit construction project of this thread namesake. Also it would utilize pre-existing projects that have more flexible scopes (e.g. GO RER, as some route completions is delayable) rather than more niche projects like UPX. One problem is politicians may try to cram new projects into the scope.
The cost efficiency of the spending is always a legitimate question, but historically has indeed occurred in infrastructure that would otherwise not occur when it did (e.g. less likely to be postponed/cancelled by a subsequent government). Even the SmartTrack "circus" (now that Eglinton heavy rail is removed) is simply otherwise useful tweaks to GO RER that are far more minor than the typical fighting between choosing a BRT, LRT or subway (political spin notwithstanding -- it is more elementary "looks like a duck, quacks like a duck, it is a duck" when it comes to RER vs ST).
Also, UPX-style mistakes being repeated so soon is historically (on average!) less likely to happen with lessons applied to subsequent projects occuring right after. Granted UPX being steered the best they can into the GO system already with "About time! Better late than never!" kudos -- with clear indications they don't want to back themselves into an inflexible corner when they begin GO electrification. The RER buildout/operation is much more flexible, not every single station/mile of RER desparately is "must finish at all costs" by expo (heck, they could delay electrifying north of Aurora if Metrolinx still have a ton of diesel trains and EMU manufacturing is delayed -- it's not going do do the expo one iota of a hoot). The Expo 2025 would simply help cement the system into proceeding, rather than a monolithic "must finish every mile & station at all costs" project. The whole electrified GO network doesn't prevent diesel Bombardier BiLevels from merrily going their way under the catenary, in the event of EMU delivery delays -- focus the first EMU deliveries as far as Kitchener electrification goes, and LSW/LSE. Flexibility better than UPX is not rocket science. But starting the GO RER buildout without delay is critical. We've really full throttle forward on a lot of rail infrastructure in full expectation of RER electrification -- Georgetown Corridor overbuilt for present service, Crosstown LRT improved interchanges with expected all-day GO lines, massive increase in Union offpeak utilization (or else it's an Auramall disaster offpeak), etc.
Delaying/cancelling RER -- one of Canada's biggest ever transit megaprojects in all of history -- is hugely costly, and likely far outweighs any real/perceived "circus" or "waste" cost of Expo 2025.
Whatever color you vote for, 4 years is not enough for whatever new major mass-transit system you prefer be installed -- particularly anything remotely resembling the biggest GO transit construction in history with potentially a gigantically huge overall cancellation penalty (combined together, whatever cost/reputation/economic/congestion/less GTHA-wide tax revenue in the next 30 years/etc) that may conceivably potentially massively outweigh an Expo "circus/waste cost" (whether perceived or real) no matter how much you hate it -- especially in Toronto's special post-almagamation situation. One can love GO RER and hate Expo, or vice versa, those are two different opinions.
Given GO RER/electrification being a long-planned project with far better overall (systemwide on average) transit case, with plenty of more-inexpensive deadline-miss cost, more-incremental, less-mandatory-rush, more scope-shift opportunities (e.g. delay electrification to specific stations/lines, reschedule specific stations, reassign train fleet around, to complete during or after Expo) than UPX which was monolithically pigeonholed as being essential to PanAm. It's a 6 month visit-anytime event, not a 2-week photobulb flash. In past expos, sometimes a transit line opened partially and finished opening before the expo closed 6 months later, without disastrous results. I admit that my statements will create replies that goes off the scope of the GO transit construction projects which I originally mentioned the expo relating to.