(not mentioning that I am not payed enough, e.g. the "software" task is payed 400 euro per week
That is entirely your problem! These things are known and expected, updates WILL cost... they must be factored in and if the customer isn't happy he can stay with his existing solution.
The problem is simple - the devs working on those things do that daily. They will naturally start by doing things easier for themselves and for whatever their own goals are and others who will see an updated dependency and care for it the same day in 5 minutes, then for new users, and only as a last priority try not to break it too much for existing users - and if you were in that position you'd likely do the same.
The open source game is simple - you get to benefit from decades of engineering work done by others for free, but it's your responsibility to glue the parts together to make it work. Nothing forces you to use it, if you're not happy you can go roll your own (
) or pay up for a commercial solution.
You apparently offered the choice to your customer to go with a commercial solution that was deemed too expensive, and improperly budgeted the alternative open source solution, which obviously even if the code itself is free does have costs - that might actually be higher than the commercial solution!
Linux/open systems are a rolling ball, as mentioned when you're working on it daily you'll see small things break due to updated dependencies etc as they come so it's a minor task to fix at this point as only one thing is likely to get broken at a time making the source of the issue obvious, and the creator of that dependency will likely have limited and documented the impact of the incremental update as much as possible. Now the longer something stays unattended the more things will break and the harder each of the fixes will be, so it gets exponentially more complex to get up to speed with. At some point it's actually better to start the actual project over with the currently available tools than trying to get the old thing to work again.
But that's something that can easily be modeled and turned into a budget estimation for a client. Count a rough amount of time you would have to spend to keep the project up to date on a weekly basis, and turn that into some amount per week the codebase has stayed unmaintained, and add that up. It should pretty much average out the simplifying factors like "lib X was already mature" and the more difficult ones like "platform was completely dropped from support so current versions can't be used and old ones have to be dug out". After 8 years that number should become so huge that it will be obvious it's not a viable option, in which case you have to evealuate others, like redeveloping the whole thing from scratch or going for a completely different solution, whether software, hardware or both.
But as mentioned if you keep an entire build setup as is in the state it was at the time of the last build so you can reuse that then you're bypassing all that. It has a cost too, so must be billed accordingly with proper explanations.
So yeah, the TL;DR is that you're somehow surprised about the obvious, have offered yourself for a job for way too cheap, but are trying to put that responsibility on people who gave you their work for free instead of admitting your mistakes