These are not two signings near the prime of their careers. These are two 30+ year old players who were out of contract (not playing) for anyone. Drastic difference. The league is not adding 28-32 year olds to add validity. It is adding 32-25 year olds. If that is your target, then you have to compare it to your competition, who are signing Gerrard, Dos Santos, Pirlo, etc. The Sounders signed Valdez and Ivanschitz.. You say this is great for the MLS, but admit you had barely heard of either of them? Surely you can see the difference. This was my concern. Either go for the bigger DP player, or look for the younger talent that can help both the short and long term. Chicago did that (as my example). Seattle did not.
Seattle is not the face of the MLS anymore (was it ever, really?). That's probably LA, and in my estimation will soon be NYFC. More people are interested in Keane and Gerrard than Dempsey and Martins (these two are polarizing figures in world soccer? Dear me.)
A young DP does not build the brand, and that's not the point of signing one. The brand itself is going to continue to slide from the top if they don't win a trophy soon. You can have all the pomp and ceremony of a military graduation at your games, but unless you win, people will stop being mesmerized by it. That's how you become a serious destination. Young attacking players (like Morris or Ibarra, for example) would help more towards winning a trophy than the two bought. Time will tell I suppose. As Smurf said far more succinctly than I, the team (league) could use some fresh blood. Does it really have to wait another 5 years. Chicago didn't.
The MLS is getting there. Will the Sounders keep up? They were gifted Dempsey and it hasn't lifted the team to anything new. You say they can do this or do that. Until they do, I'll remain skeptical.