Sketch has always sort of been an implicit subscription: you pay the current retail price, you get upgrades for a year, then if you want to upgrade past that, you pay the current retail price again next year. As far as I can tell, you can still use your old version of Sketch without switching to this explicit subscription model. So, that part doesn't feel like a huge change to me.

I assume the SaaS version phones home to check your license all the time, and probably it won't let you use the application if you aren't connected (this is speculation on my part). But (also speculation) I assume it's already been phoning home telemetry data for years, and most professional designers are online anyway. So, practically speaking, not a huge change for most users.

But, this does feel like a notable change, and not for the good. Other than its drastically improved UI and laser-targeted feature set, the thing that distinguished Sketch from Adobe CC when it came out was that it wasn't a SaaS product. Now it is.

It feels a little desperate. They've lost a lot of market share to Figma, and this feels like a pivot from financial necessity, not something they are doing to improve the product, which is where you'd hope the impetus would come from.