There are a number of fangames that prove that unscripted loops in 3D are possible, and I don't personally see a problem with simply being able to skip them by jumping through the middle if you want to. I see the issue (in general, not just in SRB2) as being more one of necessity. As iconic loops are to Sonic as a franchise, I still don't see a need for them to be anywhere and everywhere. They are at their best in places where they make sense, whether that be them being constructed into machinery or part of some whimsical landscape such as the inside of a giant donut. I've always seen loops such as the ones in Speed Highway, City Escape, and Radical Highway to be a bit much in regards to suspension of disbelief.