Your usage profile sounds an awful lot like mine is. Your headline question, can the OS be a disappointment for people like us? I'm unsure, as I'm certainly very happy with it and I largely know what I'm getting myself into when I purchase an additional phone. Using the preview builds also gives me insight into what's coming along.
Other people who aren't familiar with WM, if they've used other smart phones will find differences certainly. Will they be disappointed? If they're heavy apps users, and the apps aren't on this OS, then they might be - but if they have our low app useage profile, then they may be pleasantly surprised. No more hunting around for OS updates, everything can be automatic for them. The OS seems far easier to use than the rest too. If they changed their employment and their use varied, then it may be an inconvenience. My organisation puts out apps but only with the other 2 major systems, although they did have WM covered at one point. I get by really well and I'm not a huge apps lover anyway - so if their profile remained constant, such that they would use apps infrequently and not as a priority, then they'd probably not find it a disappointment.
I travel a lot and so being able to get my maps downloaded in advance via wifi has been a bonus since Nokia maps. I'm generally a satisfied user, albeit not the target market for MS's future growth strategy in moboile
I don't always use Edge to browser - I've been an Opera fan for 10 years+ and their mobile browsers can save on data use. They've got ad blocking and vpn built into many versions, with ad blocking in their mobile browser, so it's an added bonus.