I am old enough to remember that Chrome didn't really take off until Google released an extension framework with many popularly supported extensions. There's likely a lot of room in the browser market for custom browsers curated and managed by trusted third parties but my sense is that a) given their history, people wouldn't necessarily feel comfortable having Microsoft fill that role and b) given their size, Microsoft wants Edge to compete directly with Chrome and Firefox for absolute majority market share, rather than chase single digit % niche markets (which would be a very exciting business for almost anyone else).