Hey y'all, try to pace yourselves a bit! It's normal to get tired of any single game-- rather than giving up, give it a rest and play a different MK, or another game altogether. After a while, the fun of MKDS will return and you'll be back.
You know, MKDS is less than a year old. It's an infant. The other MK have longstanding competition and interest after YEARS. It's very possible to participate in MKDS racing for years to come, don't burn yourself out!
Regarding Nintendo hardware, folks aggressively playing MK, F-Zero and Smash Bros. have ALWAYS been really hard on controllers-- the big difference, of course, is that MKDS is on a handheld, so you can effectively ruin a processor and a screen when you take out the D-pad or R button. Still-- the "broken" DS or DSL is probably still valuable to a casual gamer (parents or siblings?) or even to yourself for playing different games (especially games that focus on the stylus for control). I think it's just part of MK Time Trials-- you WILL eventually destroy the hardware.
Oh yeah, and here's a plug for MKSC-- it's a great complement to MKDS, because it plays so differently. No snaking, no aggressive D-pad movements-- it's a much more elegant game. It has 40 tracks, you can play it on the DS, and there's open competition on the MKSC board. I've been switching back and forth between MKDS and MKSC, it's been really fun and I think I'll be doing this for a while to come.