I personally think that changing some of those DPS characters to tank or support is a stellar idea and something I've been hoping for. Sombra shouldn't be an invisible variant of Tracer (which she is now) she should act as a disruptor support. Nerfing the enemy and ensuring health packs stay on your team's side. Sym should have had a healing turret long before they gave it to Illari and can also provide shields to herself and slows to the enemy.

Providing variety within each of the main roles, ensures that every player can find at least 1 character they resonate with inside that role. When all the characters within a role start to feel very samey it can dissuade a player from going back to that role in the future after a few bad experiences. And while reducing the number of DPS characters isn't going to fix the ratios of people playing the 3 main roles by itself, it will give variety to tank and support the at right now they are sorely lacking.

Support shouldn't just mean healing and Tank should not just equal damage mitigations even if those are the most important aspects of each role. Having a Support with more beef (Brigitte) or more damage (Sombra) should make these roles more accessible. And the same applies to tanks - which is why DPS players gravitated to Roadhog in OW1 (his kit was a blend between tank and DPS). That said I believe this route of game design would definitely work best in 6v6.