@remie, I fully agree with you. My proposal above addresses the production environments only, where end-customers generate all of the traffic. In case of dev and staging environments, no extra charges should come up at all, at least not within reasonable limits (normal use).
However, there is an area, where Atlassian has not yet given us clear guidance and means: load/performance testing. Forge apps may also need this feature, especially if Forge Remote is in place, dealing with a massive workload and/or large amount of data. I think, there should be defined and provided dedicated environments/sites for such purposes (having no interference with production environments and infrastructure). Whether they should and will infer further expenses (passed on to the Marketplace partner / developer) is still an open question. I’d argue that on the one hand it is also a kind of developer environment, on the other hand it may clearly generate higher traffic and thus falls into a different category. Nevertheless, at the end of the day it is optional (I mean, it is not mandatory to create that complex apps requiring performance testing, and if so, you can still decide whether and how you want to run those kinds of tests). Anyway, if any extra costs may still appear, they should not dissuade developers/vendors from doing these tasks and provide high quality apps/services.
Btw, in certain cases, the currently applied Forge platform limits cannot really serve normal use cases either. First, it should be also defined what are considered normal and extreme traffic, i.e. what we may call a performant app. The current state of the Forge platform does not really allow for the development and constant operation of more complex and scalable apps, even the simplest use cases may reach its limits.