I'm not so sure about that. I recall my brother and I getting a bunch of really cool looking Mega Block sets one year for Christmas. There were colors and shapes we had never seen before, and neat themes like dinosaurs and moon bases. In theory it was all compatible with the absolutely huge amount of Lego blocks that we had in our combined collection. What we quickly found out however was that Mega Blocks were cheaply made. They barely locked together with each other tightly enough to build with, let alone with Legos. They were also fragile and routinely broke because the outer walls of the blocks were so thin. My parents actually ended up complaining to the company, which netted us a ton more Mega Block sets. We tried to make the best of it, mostly because the dinosaur pieces and 3D building bases were unheard of in the Lego ecosystem at the time. Eventually however, my brother and I went through every storage tub we had and purged the Mega Blocks from our collection. It was a lengthy undertaking, especially at that age, but the Great Purging left our superior Legos free of any kind of contamination. I'm not sure if Mega Blocks have improved over the years, but I wouldn't chance it personally. I still have all of my Legos in storage though, about ten big, plastic bins in all. One day they'll have someone to play with again.
Now has Lego sold out? Probably. I recall branded sets like Star Wars on the horizon right as I was growing out of the little blocks. It didn't seem harmful then, but nowadays it seems like Lego has more licensed sets than they do good 'ole fashioned knights and pirates. You can still purchase the regular old blocks, but they don't get the shelf space at the store that Harry Potter and Batman does. On the bright side, it does seem as though Lego has really tried to move away from the specialized pieces that were becoming so prevalent toward the end of the 90s. There's also an exciting global community that has sprung up around Lego, so you can buy homemade, custom pieces. I've seen weapon packs, for example. My brother and I would have loved something like that back in the day, when we were building entire Lego cities across our bedroom floors. We didn't realize it then, but we had essentially created a Lego RPG. Each character had a name and personality. Characters could only move so many pegs per turn, battles were decided vie dice, etc. Go figure. :p