Hi there,
Yes, the hardware stay the same, but software gain more features and functionalities over time which may require higher specs. And it’s true in all kinds of software unless it’s stagnant to its version. But given that your host wanted you to upgrade to VPS only means your current one has lower resources. Have you upgraded your Wordpress, themes, and plugins? I recommend downgrading it restoring the optimal copies.
Unfortunately, we don’t have control over their rules and it could change anytime. The 73% is still okay, and the question is how much CPU is given to your site and how much is in use. Because why would they give you 4 CPU (as an example) if you’re not allowed to use them all? Unless those 4 CPUs are shared within many sites on the same machine. And if there are 4 CPU and there are 4 sites, then that means 1 CPU for each, but what if there are 6 sites? That’s how shared hosting works, and it’s like don't use CPU as much as possible scenario. Though that’s just my understanding, only I could recommend for now is to switch to VPS or try another host.
Thanks!