I think the general consensus is that it improves security of the protocol, but obviously that won't matter much if the implementation gets something wrong or has worse security by itself.

Issues in the protocol itself would need all implementations to change, but issues in the implementation would obviously be isolated to one implementation. For something like Wireguard, I'd wager a guess that issues in the implementations are more common than issues in the protocol, at least at this stage.

I wouldn't say that multiple implementations are duplicating the attack surface since most users will not end up running them in parallel.

Diversity is a fantastic thing for security. It limits the impact when a bug drops and gives the possibility to migrate or run a mix of systems.

Competitions helps in multiple ways. It improve tooling, test suites, CVE response time, documentation and evolution of the protocol. There are some counter examples where compatibility suck, like DLNA but the problem often come from the spec.

That's really good because it means it will be able to have more exposure, more exposure means more improvement, more improvement eventually dig out bad bugs and reduces the attack surface in the long run

Is having Mac OS and Linux a decrease or increase in security over just having windows only?

dont fix if it ain't broken. look at sudo-rs and other rust ports.

ofc, thats a cynical view.

i personally think its a bad idea to duplicate efforts. better combine them. otherwise u risk making mistakes that were already solved. missing lessons already learnt.