I can tell you put a lot of work in, but it's still very expensive for what it is. It's something I would expect to find for free in my browser's extension store if I'm being honest.

Is there any guarantee the extension will be supported for a long time after purchase? Or is it's possible a year down the line CSS Scan gets abandoned and the extension falls out of date?

If it was a one time $5-$10 or $1/mo purchase, I would definitely buy it if I found it helped development for me.