They'll probably continue flying the JF-17 well past the 20-year mark. We can keep the F-7Ps and Mirages up, so I don't think PAC will have trouble with the JF-17.
I guess the Chinese are offering the PAF a net-new capability through the J-10CE. It could be a VLRAAM (e.g., PL-15), or something else, but I'd wager that is the reason why the PAF would be interested.
However, if the PAF acquires the J-10CE, it wouldn't be to plug a short-term need. Pakistan can't afford to think along those lines.
The J-10CE would fit into the PAF's long-term needs, e.g., complement the NGFA. The PAF could use the J-10CE and NGFA together for offensive operations, while the JF-17 is the main defensive asset.