Just do it.
I know this stuff will frustrate you in the early days. “Proof of Concept” and Paid Trial deals where the prospect, after you do so much hard work to onboard them … can just up and quit in 90 days.
But you have to change your perspective. In SaaS, you have to always re-earn your customers. That’s the point of recurring revenue. The good part is it recurs. The tough part is it means, no matter what the contract says, you have to continue to provide enough ongoing value that they renew. Even if you force them to stay a year, it doesn’t matter in the end if they end up leaving on Day 90 or Day 365. You need them to be customers for life.
Later, when you have a Big Brand, you can say no to opt-outs after 90 days. Salesforce does.
But you aren’t there yet. Do what it takes to earn each customer’s business. Whatever the outs.
If your product, team and support are awesome, and you meet their needs … they will stay.