The new Content has been developed considering the game balance, so it's not imbalanced. You still can complete and win with older Cotnent. Skill and strategy matter, so it's even possible to compete having lower Grade Content (but of course, it's not easy at all).
While this is mostly true, some of the best mechs in the game are paywalled. Eclipse, Nomad and Seeker. While it is still possible to beat these and isn't unrealistic either, they still have a significant advantage over most mechs. Them being paywalled means paying players have an advantage, and a significant one. And even if that wasn't true there's always the possibility that in the future an especially powerful item could come to the game that gives paying players a significant advantage as long as the items are paywalled. This would be a power crept item. Actually this is quite true of seeker.