This is patently not the case with Microsoft. Minecraft continued on other platforms and Minecraft Legends was released for other platforms. The statement regarding Elder Scrolls yesterday also makes clear that it's not known if traditional cross-platform games will continue to be cross-platform.
For folks with only a PlayStation and not an Xbox or PC, it would be better simply knowing up front you'll never see another Bethesda game than eternal uncertainty because nobody likes uncertainty. But from Microsoft's perspective, they simply can't provide that clarity, these are decisions that it would be foolish to make now, years and years away from games's release.
I'm generally not fussed when a console manufacturer buys a publisher/developers because all that really means for me is that my choice of which platform I buy it in narrows a bit. But my genuine concern with Microsoft buying Bethesda in particular is Microsoft's focus on selling content as a service and beyond whatever it is Bethesda are deep in development of (Starfield and probably Elder Scrolls VI) that beyond that we'll get more games like Fallout 76 and less single-player RPGs. I assume the Sea of Thieves and Grounded model is working for them.