I am not selling Plesk products, I do not work for Plesk and am not paid by Plesk. We use Plesk as the web server administration interface for users who lease a webhosting package. We do not charge extra and we do not sell individual Plesk licenses either.
My friends, you are trying to talk bad about Plesk. It's o.k., we luckily have freedom of opinion. But saying that the support of your provider sucks and then blaming Plesk for it is so obviously wrong that it already hurts. Unfortunately, Plesk license resellers normally have no or extremely bad support, although by a Plesk reseller agreement they ought to provide support to their users. Most don't, and the few who do do it in a very bad fashion as dozens of similar posts on this forum have shown. But again, this is not for a bad Plesk support. Plesk support has the big advantage, that it is not call center support by some interns, but engineer support by persons who know the product in depth.
And your argument on lacking Dovecot, Nginx or whatever support: You seem to expect a 360° Linux tech support. Plesk knows their product and will be happy to help out beyond, but you cannot possible expect a software vendor that you pay for "Plesk" to also support third-party applications in full. What we can all expect from them is that problems that are located in their product or in the interface between their product and other products are subject of support. Anything beyond, specifically OS support or support of third-party products is a bonus. Your position is like "Hey Microsoft, I have MS Office, but I expect you to support my HP printer driver, too."