I guess they figure that it's better to be a 'works' team rather than a customer team. Whatever Honda may say,Red Bull will be a works team because they are their best chance of a winner so their efforts are bound to be concentrated on them. It seems like Red Bull have looked at what Sauber have done this year and decided it's the way to go, but Sauber have still gone through more parts than they should I think...
Long and short of it is that Renault aren't really looking likely to up their game anytime soon and Merc/Ferrari engines would be a sure fire way to second class team (not that they'll supply them anyway) so RB don't have a lot of options. Seems like it's the best they can hope for until the new regulations in 2021, and maybe then they'll be able to build their own engine - which is surely the best option long term. Especially as F1 is so political, and being a supplier gives you more clout.