That's what happened once with PCs in the 80s/90s and then again with the web in the 90s/2000s. The number of developers went up. Maybe the number of developers on the previous technology went down, but I'm not sure about it. Example: there are still developers for Windows native apps. Are they more or less than in 1995? I would bet on less, but I won't bet anything of valuable.

The change in the number working on Windows native apps is probably not that important.

People learn new skills out of interest or opportunity. A lot of people working on Windows native apps started working on web or mobile apps.