There's no regulatory requirement for 3G shutdown. No CRTC plans to designate 3G frequencies/bands exclusively towards other uses.
I'm guessing the carriers will move away from 3G only after all network sites and all subscriber devices have been upgraded to support 4G (or better). Maybe earlier than 2025, maybe extended beyond 2025, but the tentative 2025 announcement is a polite "heads up" to everyone in and out of their industry that they don't plan to continue servicing 3G forever.
I imagine that some providers may continue to offer "legacy" 3G service in some areas (if they've still got active 3G machinery installed). Maybe they all will, since 3G-vs-4G offers some advantages in certain radio/terrain conditions, mobile battery life, network operating costs, etc. Maybe valuable ancillary revenue for some carriers, maybe valuable communications service for some subscribers, but this is basically niche-market stuff, it's not the Big Tech and Big Money (almost) everybody wants to buy and sell.
Let's be realistic: very few consumers will still have working 3G-compatible 4G-incompatible phones by 2025. Old phones just won't work forever, at some point the old (dead/dying) battery just can't be replaced with a new battery anymore, at some point the older operating system just can't run newer apps/software anymore - more to the point, constantly replacing/upgrading things, buying parts, tinkering, tweaking, hacking, and modding isn't for everyone and eventually ends up costing more than just buying a fashionable new (4G-compatible) smartphone anyhow.
The carriers are of course always utterly delighted to sell you a new phone.
Public Mobile will need to migrate along with Telus at some point, preferably before 3G is entirely phased out. They're already losing some business in certain areas which happen to be serviced by Bell equipment which no longer supports 3G. I expect VoLTE calling will be implemented across Public Mobile "asap" unless they want to sacrifice their market share (more regions full of subscribers) without even having a chance to fight. It's inevitable ... but it's also not something I'd bank on happening as quickly as it should.
Remember that Telus was once the technologically dominant leading carrier in Canada - all of the Big Three used to leapfrog their way to the top when phasing in network-wide upgrades/etc - but in recent years Telus has invested in other things and let the rest of their business slide into just keeping up with whatever Bell is doing.
