I don't think anyone is writing him off as a 'paydriver', but whilst acknowledging that he's a good driver who deserves another chance, you must acknowledge that he got another chance because he has a lot of sponsorship.
This. I'm not sure there was any better driver available, beyond Vergne, and I rate Vergne pretty much the same as Magnussen. They've got a really similar career actually - both did great in FR3.5, got a shot at F1 and were screwed by being beaten by extremely good team-mates, despite not being far from them. I think Magnussen would always be on Renault's shortlist if they were to give the seat to the most talented driver available, but it's pretty naive to think a couple of millions weren't a factor. I think what happened is that Magnussen was lucky that he was raising this sponsorship to try to get a Manor seat, and this came in extremely handy when a Renault opportunity suddenly opened up.
Either way what's a "paydriver" anyway? We're seeing more and more good drivers raise sponsorship packages. The extreme example was Alonso's move to Ferrari being paid by Santander, but we've got other more conventional "paydrivers" that are pretty good in F1. Sergio Perez comes to mind as the perfect example, Felipe Nasr isn't a bad driver neither, and even forum favourite Kamui Kobayashi paid for his seat at Caterham 2 years ago. To some extent every single driver was a paydriver at some stage, it costs millions to get yourself to F1 these days, the difference is where the funding comes from and how the driver is perceived to be. Of course there's subtle differences, one thing is a Maldonado paying absurd amounts of money to retain a seat that wouldn't have been his for years; or a Yamamoto type driver that is totally incompetent and wouldn't have been anywhere near F1 except for his money; another is a driver being funded throughout his career by a manufacturer or top team who believes in his talent, like we're going to see for example with Wehrlein now at Manor. The former is deemed a paydriver, the latter usually isn't called it that way. But both are paying for a F1 seat. Magnussen's status at the moment would fall somewhere in between. He's no longer being funded by a team in hopes he'll be a champion (like he was with McLaren), but he's not really that kind of driver that is being given completely undeserved breaks due to his money.