Rockies and Padres both overspent on veteran third basemen and are 2 of the worst teams in the league.
Cardinals never signed Molina to a long term free agent type deal and had not issues letting Pujols walk and were much better off for it. Turner's contract in his 30s was 16x4. Wainright and Kershaw are pitchers, and my point is that is where good teams are willing to hand out big free agent deals. That said, Kershaw was willing to stay on shorter term, option laden deal. I doubt either the Dodgers or Cards would give Rendon a $220+ million contract if they were in the same spot, and doubt either goes hard after him in free agency.
A 6 year contract means he starts his last year at age 34 (June 90 birthday). That's not that far in the aging curve, and his bat could play at 1st if he lost any range. I just don't see this as a high risk contract. Contrast that to Bryce, whose contract goes to age to his age 38 season.
I agree we are in a period of transition when there seems to be more money spent on pitchers late in their career than position players getting long term deals into their 30s, but I think some of that is due to the big spending teams having a lot of young position talent coming up at the same time. I also think that the conventional wisdom that smart money doesn't chase pitchers long term over 30 is still generally right, Verlander and Scherzer being huge exceptions. Lots more Prices, Sales, CCs, etc... than there are $150MM+ contracts for pitchers that work out.
You take the Nats where they are. They can probably stop gap at 3d and upgrade at 1st to remain competitive next year or two if they don't sign Rendon, but a 6-7 year commitment at Arenado's AAV probably isn't stupid money.