There is probably a huge chunk of people who only need a slither of the features of a SaaS but are paying for everything they don't need.

If your particular use of a SaaS is not susceptible to security issues (for example you can use it on your local laptop) then a slither of features that are insecure is exactly the thing for you.

Not everyone is going to replace SaaS with half-baked personal implementations, especially the big companies that most SaaS are aimed at, but it will gnaw out some of the long tail of SaaS subscription revenue for sure.