I agree with you about the effects and would prefer that cannabis just fall quietly out of use. However, some observations based on having lived in a few places (in Canada and the USA) where it is fully legal:
Cannabis sold legally is actual cannabis; I personally dislike the smell but it's just a strong, pungent herbal scent which some people even like. When I visit places where it's still illegal, I constantly smell horrible skunked weed everywhere. I do also still sometimes smell this in places where it's legal, but not as much. A key factor is price; if the price is allowed to inflate as most prices have here, you'll have a brisk illegal trade in cheap fake weed that stinks. If the price is regulated - or even more effectively, subsidized - people generally prefer to buy the real thing from a legal source.
Legalizing doesn't really make a difference to medicinal and therapeutic use (except in cases where many people are uninsured/don't have public healthcare, which shouldn't be the case in the UK) because prescriptions were fairly liberally granted and very easy to fill. It mainly makes a difference to recreational use.
Driving under the influence was initially a big problem in the US, mainly because individual states rushed to get legalization propositions on the ballot as soon as they felt they had a majority of voters on their side and didn't work out all of the details first. Now both the laws and the technology are (mostly) in place and it works pretty much like DUI for alcohol.
Official cannabis retailers are everywhere in my state and they have physically replaced a lot of other business and services. I suspect the pre-existing places would have gone under anyway due to the economy and effects of COVID, but perhaps without cannabis legalization they might have been replaced with more interesting and diverse businesses.
This is purely observation, but I think the interest in recreational cannabis has overall declined here, at least among adults. I literally never see anyone in the cannabis retailers besides staff, and the parking lots are always empty (of course, that may be because there are so many places that the business is spread out). In upscale communities even the teens consider cannabis downmarket and gravitate to cocaine and other "study drugs". College students, the "buy nothing/boycott everything" crowd, and survivalists still like weed, but the latter two categories generally grow their own.