As much as I despise chromium controlling the browser market, having microsoft in the game should improve the situation, so kudos to them.
Monocultures never improve anything.
Yes there are problems to everyone using the same software but there are advantages too. One drawback is that potential security issues would affect "everyone" but on the other hand the same fix works for "everyone".
If the common software is open source it serves as de facto standard/reference implementation so you don't have compatibility issues. Standardization processes try to address this without reference software but IMO that is a lot more effort (in terms of engineering hours) to get it right so you can have a fully featured standard that is clear, has no bugs and has multiple perfectly standard compliant independent implementations.
After all software is just a tool to serve a purpose and our decisions in this area should be driven by pragmatic reasons alone. If we can serve the same purpose with much less effort by having everyone build on top of/use the same common base why not.
Similarly I'd rather see everyone just use Linux instead of all the different popular OSes we have.