completely agree but it could be added that a new language can sometimes help explore new ideas faster, in which case maybe the routing layer and protocol can see new optimizations

It’s important to remember that safety is the whole purpose of the thing. If Tor is slow, it’s annoying. If Tor is compromised, people get imprisoned or killed.

This is not correct. Tor is generally not bottlenecked by the quantity of available nodes, the usual bottleneck is the quality of nodes picked by your client rather than the quantity of nodes available.

Of course, technically, this problem is related to the quantity of high quality nodes :)

Yes, but let's not forget it's voluntary based. There are lots of high quality nodes, although less which are basically burning money and getting nothing in return. We all believe in a censorship-resistant and free web but only few are willing to take action. My two small guard/middle relays are rented at 10$/m each and is only 100Mbit/s non-metered up/down because it gets expensive.