People are overreacting to this rumor honestly. More importantly this rumor has yet to be confirmed, but is seen as tantamount to Sony announcing it themselves. Also, please remember that Horizon is nearly a 3 year old game and has exhausted its legs on PS4. It potentially arriving on PC is being conflated with all PS exclusives arriving on PC, which depending on your perspective is less of an incentive to buy PS5. Personally, I think there's nothing inherently wrong, or bad with exclusives arriving on the PC, and is a way for developers to further generate revenue for their properties and expand their player base and ecosystem. However, I understand the perception that if first party games are arriving on another platform, that will potentially outperform your next-gen console, what reason do you have to get that console- and it is that sentiment that leads me to think we won't see exclusives launch concurrently on PS5 and PC, if they arrive on PC at all. PS5 will get them first, of course, and some may arrive at a later date. However, I think it'll be a somewhat arbitrary system, with no clear pattern in terms of how long games will take to arrive, so Sony doesn't encourage players to opt to wait for the games to arrive to PC and not buy the PS5.
The closest analogy I can think of are the latest Google licensed phones receiving Android OS updates first, and then they gradually disseminate amongst other flagship devices that they'll support. So "loyal" customers get to experience the games first, and others that aren't gaming on PS5 will possibly get them in the future. When you look at the success of MH:W on PC and the support that title has received from people on that platform, it makes sense that console manufacturers would eventually adopt a similar stance, so long as their games aren't releasing for their direct competitors, it doesn't harm their primary business. Also with Death Stranding using the Decima Engine and arriving on PC, it became incredibly easy to port and profit, with minimal work, Horizon over as well.