Why isn't this using an ImageEffect Plugin, but RouteSubscribers?

PS: No bad intentions, just trying to help, as I saw this when reviewing the modules code.

We know about the plugin system.

The idea behind our module is that calling the TinyPNG API should be the very last step.

The optimal solution would be to catch that point when a new image derivative has been created. Currently there is no way to catch this. There are some Drupal core issues about this problem, i.e. ✨ Make possible to respond to image derivative creation Needs work ✨ Split ImageStyle into the config entity and a separate event-based image processing service Needs work .

When this approach will be possible we will replace the route altering way.

Thanks @tikaszvince so you say this is done by design and won't be fixed by using an ImageEffect or ImageToolkit Plugin right?

The idea behind our module is that calling the TinyPNG API should be the very last step.

Wouldn't that also work, if it's just configured to be the very last effect used?
That could be validated by a validation callback maybe for example. I think that might be more straight-forward and the Drupal way?