Okay, let's me phrase it this way then. It's a big piece of weird code that isn't intuitive at first but become relatively easy to work with once you understand what's happening (give up on the why, half the stuff makes no sense). The current version is going to crumble, but hopefully Magento 2 will be built to last.
Not really accurate to say that Magento is "built on top of... Zend." We don't use much of it.
Regarding the crumbling comment... well, Magento is seven years old. There's a reason we're releasing Magento 2. Code's on GitHub, PRs welcome: http://github.com/magento/magento2/
Magento is a monolith and always will be. It's built on top of PHP's slowest framework, Zend which doesn't advertise speed but rather raw power.
Magento is going to crumble, it's just a matter of when