I’ve lived in 3 houses with conservatories (all in place before we moved in).
The first one was really just a glorified lean-to, old, cold, full of spiders, and definitely would have been replaced if we’d had the money.
Next one was huge, across the whole of the back of the house. Great space, bright and airy, but with a persistent leak that just seemed impossible to fix. We could use the conservatory most of the year though as it had lots of doors and windows in summer, and a fan, and two big radiators in the winter. After we sold that house the new owners pulled the conservatory down and put a proper brick extension in its place.
House 3, which we’re in now, has a 3x2.5m conservatory. We’ve removed the doors leading into the kitchen (not planning to move anytime soon so no worries about building regs) and put in internal bifolds. The conservatory itself is really well built with a good solid structure. It cost £25k to put in 15 years ago and is still holding up well. No leaks at all. We put in underfloor heating and a radiator, put reflective film on the roof and spent about £1k on blinds. We use it all year round now, it’s like our daytime living room. There’s maybe the odd really hot summer’s day when it gets very hot but after 5 mins of having windows and ceiling windows open it’s fine to sit in. On days like that I’d rather be outside anyway. Nice sofa, tv, big fluffy rug. Lovely view to the garden.
So you can make conservatories work if they’re good quality in the first place. If not, and you want to pull it down and replace with a brick extension, we were quoted around £30k (SE).