The big problem is that tomcat is not stealthy. For that same reason, china needs a more modern plane than j-15.
I still see a lot of utility in something like a modernised J-15 and dedicated EW. J-35 will have shorter range and payload compared to J-15s. J-35s also will not be able to carry any long ranged A2A missiles that are designed to target tankers, AWACS, and larger aircraft like bombers. Similar to what the Phoenix was designed to do against Soviet bombers with air launched anti-ship missiles. There really isn't much preventing a flanker from receiving serious electronics upgrades to the point they approach dedicated 5th gen designs which took those electronics in mind from the ground up. The only downside is stealth but it makes up for that in providing utility elsewhere; range, payload, and greater range of ordinance available for carry. To be honest stealthy 5th gens are very limited in what they are tasked to do - either air superiority force multipliers or very limited strike tasks (tiny bays = 2x bombs etc).
Consider also how many nations have stealth naval aircraft for PLAN to have an absolute uncompromising need for huge fleets of J-35. India won't for decades and they dream about blocking PLAN from Indian Ocean access.