If you're riding for exercise, it's all the same. A worse bike is going to give you more of a work-out. That's the fundamental difference.

If you're commuting or going long distance, and you find yourself exhausted by the end of it, you might want to consider upgrading to a hybrid or even a road bike (depending on the quality of your bike trail). Thinner tires, lighter frames, and more gears will mean less effort peddling. That makes the ride easier and allows you to go farther without tiring or sweating as much. EV Bikes are also great if you do deliveries or need to go long distances / high speeds in a hurry.

I've had a Trek hybrid that I've been using for the last... 15 years, I think? It was actually my brother-in-law's bike before he moved, so the vehicle is ancient. $50 for a tune-up every couple of years and it runs perfectly well. So even if the initial price tag is intimidating (this one was in the $500 range, although I've seen vehicles go up north of $2500 back when the bike market was particularly tight) a good bike will last you a lifetime.

But, at the end of the day, the real question you have to ask yourself is whether you like your MTB. If you're uncomfortable or you're getting winded riding it or bits of it are falling apart on you, then absolutely. Upgrade. But if you're not really having an problems with the bike right now, you're likely not going to see a big different in a higher end model.