It depends on what you want from your purchase.
If you're a 12 owner, the only gain going to the 13 is battery life, and promotion if you go for the 13 Pro. Some people may not think that warrants an upgrade, others do.
If you're an 11 Pro owner going to a 13 Pro, you are gaining new design and promotion, and slightly better camera. Again, not really a big upgrade for some.
For Xs users, it's a big upgrade, especially in battery life and camera. For XR/11/older owners, going to the 13 is an even bigger upgrade as it gives OLED higher res screen, much better camera, new design, ultrawide for XR and older users, etc.
Every iPhone releases are never targeted towards the previous gen owners. Apple knows that people are keeping their phones longer and longer. So they are targeting 2, 3 generations, or even older iPhone users. Considering Apple is still supporting the 6s, it shows that Apple knows there are still many people using those older phones.
Imo the 13 by itself is a good release, ignoring the bugs from iOS15. Battery life is stellar, and it gives the 13 mini a more reasonable battery life. If I were in the market of a new iPhone, the 13 mini will definitely be on my list. The iPhone 13 is a much better well rounded product than the 12 (reduced battery, 5G hype).