Hmm, I personally never found DKC2's bosses particularly scary or threatening at any age tbh. In fact, I always found the bosses to be the weakest part of the game. The platforming was always the most difficult and challenging (and fun) to me.

However, I don't necessarily agree that old games had harder bosses or harder gameplay. A lot of the difficulty in older games comes from bad game design, poor coding, bugs, or intentionally making things unfairly difficult in order to extend the life of the game to prevent people from beating it in a few days and returning it to the store afterward. So while they were hard, it wasn't because they were designed with the idea of difficulty = accomplishment or fun. To make something that is both hard and fun without relying on bad mechanics or cheap tricks is in itself very difficult. Most games today focus more on having a good gameplay experience and bosses that are fun to fight, not necessarily difficult.

That said, DKC2 is a beautifully designed game.