If you reserve workload x on hardware y for n years, you're effectively strapping yourself into a sure-to-be-obsolete and more expensive platform which you'll have to then move off of at an arbitrary point n years in the future.
If you don't move, you wind up paying a premium to be stuck with the obsolete / more expensive platform just to avoid the cost of migration.
RIs are a lock in.
Also bear in mind that the savings you get from RIs will generally outweigh the reduction in price of the instance.
There is one truism: cloud costs always seem to grow faster than revenue. :-)
Second, the underlying financial principle is that with visibility comes lower volatility comes lower cost - that's some very basic financial logic that's at play here.
Yes, of course the contract implies a degree of vendor lock-in, but this is inherent in the nature underlying operational costs.
"RIs are a lock in." - of course. And if you don't want to be locked in, then you're going to have to pay a lot more: AWS, GCC it doesn't matter, it's the same financial reality everywhere.
Second - because newer things are available, it does not mean that others become 'obsolete' in three years by any means.
The vast majority of business do not need to have access to the latest, specific version of what are essentially commodity bits of hardware and software.
It's not really a risk for most businesses, and if it is, then obviously they can pay a higher price for the volatility inherent in switching at any time if they so chose.