This is good news for XBox Game Pass subscribers. That means that you will have access to those titles for $9.99/mo. I am not sure how this will play out for future development, as previous publishing deals through Microsoft have been wrought with problems (Mechwarrior, Vanguard, Asheron's Call 2).
I think it is Steam and GOG that have the most to fear from this acquisition. I would not be surprised if Microsoft prevented those ecosystems from selling Bethesda products, similar to what EA has done with its AAA launches. Only time will tell, but Microsoft has a vested interest in ensuring the next Elder Scrolls or Fallout game is a big hit if it wants to maintain its dominion.