how is onion messaging on the lightning network incentivized?

BOLT4 defines onion routing for messages and payments in the lightning network. This is really cool. However, I'm wondering how message routing is incentivized? Payment routing seems to be clearly incentivized as each node hop gets to collect a fee, I just don't understand how the messages are incentivized.

It seems like onion messaging is starting to be used for more and more things. BOLT12 uses it to exchange invoice_requests and invoices. bLIP32 proposes doing DNS over onion messaging. As usage of onion messaging continues to grow, at what point will the current model for forwarding onion messages get abused and fail to scale and no longer work reliably? The TOR network seems to suffer this problem, it is a free service and there is no incentive for running a node, so it is very slow and unreliable because many people try to use it.

Onion routing is only taking place while transporting payments, so it is incentivized by the routing fees. AFAICT all remaining communication is gossiping, which is necessary for all clients to stay up to date with the changing state of the network. So receiving this is in the interest of every node. Now one might think about withholding attacks where a node doesn’t forward info. I’m not sure if there have been any measures implemented to protect against that, but I also don’t believe there is much to gain from such behavior, since this communication isn’t costly and the information would simply be routed around the attacker. It might become a problem if many nodes do it, though. Not sure if anybody has looked into this possibility.