I’ve been into MTB’s for 30 years and only had one bike during my first couple of years of riding. Since then, I’ve had as many as ten and currently have 5 MTB’s and 2 road bikes. There are days when any of the bikes is the most suitable and I love riding them all.
About 4 years ago I started thinking I only needed one and built up a Surly Karate Monkey with a set of MTB wheels, a set of touring wheels and a rear single speed wheel to match the MTB front, rigid and suspension forks, drop bars, risers and a Jones bar for it and it was good at everything I asked of it from long road miles to local (mostly Dark Peak) mountain biking and even a few MTB events. The problem was that every time I wanted to just grab it and head out, it wasn’t in the set up I wanted for that ride on that day so it went the way of so many other bikes and got replaced.
One bike will work for people who like to do the same kind of riding for the majority of the time or are happy to compromise on some rides as the bike is perhaps not the ideal one for that day and that ride.
At the end of the day, it’s no bad thing to have choice. Unless you’re a serial ditherer.