I'm also firmly in the camp of never trade multiple valuable prospects for a reliever, but at least in this specific case, it looks palatable from Houston's perspective. I could be way out of my league here, but Velasquez seems a notch below McCullers and has some more question marks. Oberholzer is filler, and Fisher is from a position of depth.
It's not ideal at all to trade a Velasquez, but this is also nowhere near what Boston got for Kimbrel, and Giles is way less expensive, increasing his value quite a bit. It's a good trade for the Phillies for obvious reasons. Maybe not the best they could have done, but there aren't a ton of teams pursuing a closer, and they tapped into one of the teams with a more quality prospect depth to spread some more youth into their system.