It's an interesting thought experiment to consider if (1) you expect the Mets to blow everyone out of the water financially, and (2) Soto to (more or less) take the best offer, how do you try to be competitive if you're Boston? Leaning on Ortiz, fandom, etc maybe gets you a bit, but I agree with the general sentiment here that it's hard to see Soto leaving substantial money on the table just because of that.
I think if it were me, I'd be looking at deal structures that are appealing to Soto. I.e., maybe you can't match total dollars over the 12-15 year contract, but maybe you're willing to front-load more than others, so that the pre-opt out amount is higher. I.e., if the mets are offering (say) 14/600 (~43m AAV) with an opt-out after year 4, but want it pretty flat over the years, would Soto prefer 4/200 (pre-opt out) + 10/350 (post-opt out)? That's 50m lower (14/550, or 39AAV over the full lifetime of the deal), but 30m more over the first 4 years. If his plan is to very likely opt out, then the lower deal is potentially the better long-term bet, despite the lower total value, while still providing a lot of insurance for injury/performance/etc reasons that might make opting out unappealing. And I could see that being more appealing to ownership like Boston's, where you accept a higher AAV in the case of an opt-out, in exchange for a lower AAV if the deal ends up running the full course.