You're overestimating the Demand for retention and underestimating the Supply for retention.
In the past half dozen years there have been seven trade deadline 25% retentions. The cumulative AAV retention in those seven trades is $8.6M. There are currently six teams this season with $10m+ in free cap space--more cap space than all of those previous retention deals combined. At this season's trade deadline there should be somewhere around $300m in unused cap space that will expire at the end of the season.
Last year we even saw the playoff bound Florida Panthers retain 25% on Max Domi so he could join Carolina.
Of that 300 million I would bet 80/90 percent of it belongs to 5 or 6 teams.
Also seeing many teams (more than previous years) are kind of tighter against the cap. (Which is where your point about the owners getting more than the 50/50 split having extra money comes in)
My point was less teams available to do so and more teams may seek a retention partner to complete a trade.
If many of the “proposed” deals go through there should be more retention than usual but it may come at a higher price. … or teams may refuse to make deals.
If Chicago had proceeded with rumored early season plans to deal Toews, Kane and Domi retaining the rumored 50% on each. With a partner retaining 25% those partners would = 1.25 mill of retention. Roughly 1/6 of your 6 year total. but more deals have already been done and are proposed that have or need double retention.
It is my opinion that some teams will pay more and it’s possible a deal or two doesn’t get made because a team won’t pay it.