Subdomains absolutely have value! It doesn't matter that we have those tools "today" (it's not like we didn't have reverse proxies many years ago), different do

Subdomains absolutely have value! It doesn't matter that we have those tools "today" (it's not like we didn't have reverse proxies many years ago), different domains are still the only way to truly split up traffic. A reverse proxy by definition doubles your total bandwidth requirement, which is expensive and inefficient if you aren't running everything in one datacenter. You'd need to not only get servers with enough bandwidth to cover their traffic, but another server with the sum of all individual server bandwidths to handle proxying. And if you want to do geographic sharing, you either need to shard all services, or be fine with an increase in latency for non-sharded services.