A legal page like this is probably not the right place to be opinionated. Still, please consider:
As far as we know and see: Any SLA will not prevent you from having downtime in reality. A service level agreement is just a set of rules on what will happen when the downtime has happened.
Any regular SLA for hosting will not cover the costs of your lost business. It only will cover the costs for not providing the service during a downtime. In other words: When you pay 300€ a month for hosting and your website was down for a day, you will get back 10€.
Also, from our experience, there are many grey areas in hosting. We, for example, have different services. Let's say deployment is down, or our Dashboard is down - how is that measured? Or there is an incident making the website respond slower as usual. Sometimes it's not easy to measure and track a downtime. For example, a certain monitoring of our system is constantly reporting back "200 OK", while a Pingdom alert from a client might paint a different picture of the same situation. Our SLA (like any other) defines that we are responsible to identify what ever is downtime or not.
Actual downtime clients are experiencing is often caused by their own "coding mistakes" — by not making proper use of the available resources. Think slow database queries for example, which might work while testing, but not in production at a certain point of traffic. In most cases a "500 server error" is not a hardware problem with the hosting but a runtime error in PHP caused by the specific code of the client. Such errors can be resolved independently by analyzing the logs files, which clients have access to. We are happy to discuss and research such problems together with our clients.
Also, there are many technical measurements which can be taken by the developers to dramatically lower the chances of having any downtime at all. Our multi-node production plans are offering high availability when correctly implemented. For CMS systems such as Craft, it's often possible to cache the whole frontend in a CDN by using edge caching with a third party service such as Cloudflare.