Unity is pursuing cloud because there is such a large opportunity. Even in the world you imagine where WebGL games take off multiplayer net code, matchmaking, server hosting, live ops, etc. could remain large markets.

Today, most large game companies do many of these things in-house — by offering services to solve some of these challenges Unity is hoping to lower the barrier to entry for smaller developers to compete and convince the larger ones that they would be better off using managed services instead.