Because Boston would likely lose a 4th rounder for nothing. Matthews would need to sign offersheet to make it effective.. there is a reason why offersheets dont happen in this league. Also STL wouldnt afford Matthews if this offersheet hunt is succesful and Leafs can afterwards offersheet McAvoy if it happens which Boston has no room for. Idea of offersheets is stupid. Your idea is creative but not realistic at all.

A player still has to sign the offer sheet. Excellent chance that a player doesn't necessarily sign an offer sheet with a team with a highest offer. He still might like his situation with his current team and will accept a lower offer because of various reasons.

I’m gonna respond to your every point one by one. Trading the 4th away in exchange for STL offer sheeting wouldn’t result in them losing it for nothing. It would only help them. You’re crazy if you think Matthews won’t sign 14 million. Realistically Toronto would match any offer so actually getting Matthews and hurting STL cap isn’t a worry. I think when you say STL can’t afford Matthews you mean we don’t 100% own every first for the next 4 years but this was just an explain. Also the point of this is that St. Louis gets any and all retatillation from Toronto and not Boston. The transaction could even be as hidden as Boston trading an extra 4th for a trade they would already do like maroon. I personally believe there are a ton of loopholes in the CBA that would get abused if GM’s knew. Another loophole I’ve thought of is what stops a player like Erik Karlsson from signing a 7 year 1 million contract and then he has a second income guanteed from another company of 10 million for 7 years. The NHL contract would only count towards the cap

I wish you had shown a 2018/19 Blues roster with 14m Matthews to show how realistic/non realistic such a signing would be. Certainly it appears the Blues or just about any team is in no position to offer 14m to any RFA.
Blues only have one costly expiring contract Boumeester at 5.4m, hardly no enough to cover the difference in Matthews proposed contact.