If we get rid of Marner and spend 12 million on goaltending, defense, and depth scoring, we would almost certainly experience a net loss in quality. Marner brings greater impact than his contract, while mid-tier expenditures in goaltending and bottom six depth are some of the least cap efficient and least reliable pieces that exist. Did we not learn anything from this past offseason when we had 20m+ to spend and walked away with a whole bunch of things ranging from meh to disaster? Cup winners come in all shapes and sizes, but even the teams that do excel due to goaltending/defense/depth scoring usually don't do so because of significant cap allocation to those areas.
Winning a cup with goaltending tends to come in two forms. Either fluking into a top tier goalie in the draft a decade ago, or fluking into a hot streak from any random goalie at the right time. Money won't get us either of those things, and big cap allocations to goaltending usually just end up as a barrier to success.
Winning a cup with depth scoring tends to come from depth that is still cheap. Either ELCs/young players developing beyond their current pay, retained players, or some random cheap player hitting a hot streak. Money won't get us those things.
And while less so than goaltending and depth scoring, there's also pretty weak correlation between defensive spending and defensive results; especially through any given 20-30 game stretch. We have enough cap space to match the defensive spending of cup winners.
In past years, we have been able to have good defense, good scoring depth, and good goaltending, with our top players taking up a higher percentage of the cap than they will through the majority of their upcoming contracts, and without many of the advantages we will have moving forward. How do teams actually get good in these areas? Good drafting and good decisions. How will we improve back to that level? With our collection of quality ELC talent coming up through the system, and with good trades and signings. Not by sacrificing our best players and then throwing money at problems.