Deciding on tools for workflow - SaaS, on-premise vs Cloud etc

I try to ensure that the tools in my workflow are either 1) installable software that I pay for one time, then run on-premise 2) if cloud, then from a reputed SaaS company that's been around for a while, and I try to get "grandfathered in" ASAP.


For example, for TTS I'll use Google API because I know it's here to stay, and most other apps I see on AppSumo are just building wrappers around it. Same holds true for many AI writing apps.
I also like tools that have payment plans structured around use frequency (number of API calls, data usage in proxies etc) instead of flat rental plans, since I have some control over usage.

Which brings me to my question:
When buying tools that you want to integrate into a workflow, how do you audit them?

• Do you plan for obsolescence (ie you're going to switch to another tool in a year anyway, so might as well get the monthly plan) - - in which case, how does that impact your process/SOP management (which will need rather frequent updating if the tools and workflow keep changing). If this is the case, how frequently do you update your SOPs?
• Do you stick to barebones tools and workflows built around things that likely will always be around (php CMS, SQL databases, csv, HTTP requests, OpenCV for computervision etc)
• Do you try to get Lifetime plans for SaaS tools (in this case, since the caveat is always 'its the lifetime of the app, not the user', what cutoff do you use to ascertain if a tool/app is 'likely' to stick around?)
• Do you prefer using a tool locally/on your own servers or using cloud solutions offered by the SaaS company?