Canada can't field half of pros in his line-up, this is one of the weakest teams at RWC.
You really know dick all about Canadian rugby don't you? Pretty much all of Canada's first XV has experience playing top flight rugby...
Cudmore, Pritchard, Smith, Evans, Fairhurst, Sinclair, O'Toole, Franklin, Phil Mackenzie, DTH, Kleeberger, and Monro all have experience playing (or are currently signed) in the Celtic League, Aviva Premiership, Top 14, or ITM Cup. Twelve players in their 30 man squad play or have played top flight rugby. That's comparable to a lot of tier two nations. Also, the players that do play in Canada play in the B.C. Premier League and Canadian Rugby Championship, both of which are RFU Championship calibre.
On top of that, Canadian rugby players have passport issues that pacific islanders and tier two European sides don't have to deal with. I assure you, pretty much every player in Canada's squad is capable of playing top level or RFU Championship rugby.
Canada does have the talent to beat Tonga, contrary to what you say. In fact, Canada has never lost to Tonga at the RWC. 3-0.