back to the OP, 'one bike for most purposes', that should actually read, ' one bike for most purposes the purchaser intends to put it to' ie, if you are into touring and time trialling, then this would be difficult, but a lot of folks aren't that diverse, I'm not, Olive does EVERYTHING I want it to do, in a way I thought it would, ie long day rides, commute, audax and rough stuff, quite a bit of rough stuff, and carrying loads. 1 bike does all I want to do. I have a full sus MTB, and have used it twice since buying Olive, my Pearson Audax bike ...once. in 20 months. So the answer is yes, but it depends on your parameters, I mean lets be realistic.
If you only WANT 1 bike, and your chosen machine ticks all of YOUR boxes, then yes, but if you race road, race dh mtb , tour, tt and commute then I would say no.