But, I certainly don't use it to pressure the customer.
This is probably a bad assumption. A change in the offer price does not reflect on their trustworthiness. You’re also tricking yourself because someone wanting to lock you in down the road and raise the price won’t do it up front, they’ll do it later.
An offer price can change due to market conditions. The statement that price can’t be guaranteed is an optimistic speculation that sales or costs might jump, so you can buy now before their business increases and prices rise due to higher costs or demand. You can legitimately benefit if you buy before prices go up. You certainly can gamble that prices won’t go up, but if they do and you waited, it doesn’t mean the other party can’t be trusted, it means you lost the bet.
If you wait and get a higher price, that could even be a sign that the company is more trustworthy than if the offer doesn’t change. But of course there’s no guarantee. And ultimately there is no guarantee that when you buy a service on a limited time contract that you won’t have prices jump later when you’re ‘locked in’. It would be quite silly to not expect the renewal price later to rise for a variety of reasons, and you should have contingencies in place and be willing, able, and ready to switch providers.
I don't know what companies you are working in, but if the service is deemed valuable enough really nobody cares about money as long as it's in the ballpark of what similar services costs. And When it comes to time, yes all projects starts with "we gonna start using this this week" but we also know it takes a year to roll out even the most basic tool in a medium sized organization so a couple of weeks here and there doesn't really make a difference either.
All that matters is if the tool seems to solve your problems – and the price is not significantly different from the competitors and having a sales person bugging you every single day telling you you need to make the decision now because otherwise who knows what might happen to the price – honestly they can just go ** themselves. I make the decision when I'm ready, and if they are deciding to significantly jack up the price in between that period they are just making themself less trustworthy.
To your point perhaps, one thing large organizations value above most things is predictable cost. They actually don’t want usage based pricing because it’s unpredictable and could spike, even if the usage based cost is significantly lower on average.
I tried marketing usage based pricing to companies for too long before realizing that it was a disincentive working against me and that price didn’t matter much.
This is one of the primary reasons why I avoid any SaaS that I might actually come to depend on. It's a trap that is an inherent part of that model.