IMO, the mindmate is separate and distinct from the quality of the LTR. The LTR either has the ability to regain some life with TLC from both parties, or it doesn't. Of course if one of the parties gives up, the LTR isn't going to work. But the LTR itself is separate and distinct from any 3rd party.
I can see that some LTRs just don't have it. THey were a bad idea from the get-go and then circumstances (kids/money/whatever) made it survive for longer than it would've without external pressure.
IMO when an enticing not-so-baggage-laden 3rd party enters the scene it only messes with one's ability to step back and examine the 2-party relationship.
I think involving a 3rd party to evaluate the quality of the LTR is a dangerous and win-lose situation at best, likely a lose-lose situation.
Any LTR needs to be evaluated in and of itself without the real fantasy of something different that's an option to walk away from (which most certainly would come with its own baggage in time just like anything else in this life).