In terms of headline talent leaving the Premiership, the main ones would be de Klerk, de Jager and Faletau (although you could argue he’s no longer at that level and hasn’t really added a great deal to the Premiership).
There are some very good players leaving like Thomas Young and Nathan Hughes who have been big contributors to the league in the past, but again are probably at the tail end of being truly influential. Arguably, they’re being replaced by young English talent too. Barbeary isn’t a like for like but fills the vacancy in Wasps’ back row and Harding has already established himself as a very promising replacement for Hughes.
There are also some good players coming in to the league and even if I don’t personally rate him that much, Pollard is definitely a big name arriving.
Ultimately, losing someone like Antoine Frisch who has looked very good, but was playing D2 (or D3) rugby not that long ago, isn’t indicative of a talent drain IMO. A better example would have been Vaea Fifita, but his move smacks of mercenary behaviour.