I used to play so much MTG. I started around 1996 / 1997 and stopped playing around the Time Spiral expansion years. I don't know what's happening in the game anymore. But sure it's a very fun game. Maybe I'll pick my cards again one of these days. :)
It depends on the format you're playing. In standard a set is legal for about 18 months before rotating out. For example Origins (the set discussed in the article) rotated out of standard at the end of september this year. Modern is an eternal format where all sets from 8th edition and later are allowed, except cards put on a ban list. I'm pretty new to Magic so I've stuck to standard and casual kitchen table formats so far. Modern and most other eternal formats (except pauper) can be prohibitively expensive to get into.
The ban lists for the different tournament sanctioned, eternal formats seems to be updated regularly, so there's definitely a chance that your cards could become obsolete. I imagine it must be a nightmare trying to balance the thousands of cards of varying power levels in those formats. I agree that magic can be really daunting to get into when you see people spend $300+ on cardboard even in standard. Fuck that! I've been lucky since I have a few friends who were willing to dive in with me on a budget. Outside of drafts which I try to go to once a month I've not spent more than $25 on boosters this year. I would never be able to attend an event with my deck and be competitive, but I can still have a fun night with friends. Guess I'm not a Spike. :)