The architecture is already there with I2P, it has proven to be efficient over I2P, Tor already has extensive use as an anonymous web browsing tool, whilst adding relays to each client would help mitigate Tor's network constraints somewhat, it would still degrade performance on the network to a degree due to the nature of how it anonymizes data.
There's also the issue of not being able to run a client and a relay AND remain completely anonymous, as I posted earlier.
The I2P network is also being underutilised at this point, it has a lot of potential for growth especially in this area. It makes sense to use that potential to grow a specific specialised network, rather than potentially impact an already perfectly functioning network that Tor currently is.