^I'd say that "established" is a big word. I don't see any proof that a third party "core" game gets particular legs in sales just because it's on the Wii.
I pulled the top 30 sales on Wii for the 13th of March. Let's see only the third party games with more than 12 weeks of sales, and let me - sort of arbitrarily - try to extract the "core" games (I'm ready to accept corrections on my choices here):
9 - Guitar Hero World tour: w20 31k
15 - CoD WaW: w18 25k
18 - Mario & Sonic Olympic: w71 21k
30 - Guitar Hero III legends of rock: w72 11k
So we have the Guitar Heroes, a third party game with Mario and Sonic in it and a wide "casual" appeal and CoD WaW. Basically the only interesting case is CoD, and I can see it showing legs as basically the only good FPS multiplayer on the Wii. So while the Wii keeps gaining sales mostly because of its "widened" market, what happens to the black sheep in the family (older brother, father) that would like to play an online shooter on it? She or - most probably - he goes for the branded, well reviewed CoD. Exactly like people went for Resistance 1 on the PS3 beyond the actual appeal and quality of the game when less choice was available in the lineup... the lack of legs of R2 vs R:FOM says a lot on the subject.
So far, I don't think we found the "legs". Maybe I'm doing it wrong... Maybe I should take the third party "core games" and see how they sold...
No More Heroes
Total: 390K - 72k 44k 41k 21k 17k 14k 11k 11k ...
50% of total sold in first 4 weeks!
Okami
Total: 210K - 35K 18K 13K 8k 7k 6k 4k 3k 3k 2k ...
50% of total sold in first 10 weeks
I actually expect longer legs on ports, as people who already had the PS2 version is likely to wait for a lower retail price before buying
Red Steel
Total: 1090k - 143k 37k 31k 33k 32K 17k 13k 10k 10k 9k ...
Obviously front loaded being a launch title, but dropping fast enough after the 6th week
RE: Umbrella Chronicles
Total: 1260k - 238k 114k 66k 66k 55k 58k 47k 43 27k 24k ...
50% of total sold in first 6 weeks!
There are a few cases I could find of real legs: Sonic and the secret rings and de Blob. For Sonic I guess the name had a part into selling it to a fraction of the "widened" audience in the mid-distance
The real case is "de Blob", but it looks like an oddball rather than the rule: an indie title that gained its sales by hearsay like happened for, say, Geometry Wars.
In the end I don't see much evidence for the "Wii legs" for third party titles that can't be explained by the very same mechanics that work on other consoles ( name like Sonic or Halo, lack of choice in lineup as for CoD:WaW or R1 )
Different analysis to prove them are well accepted.