There's a whole science around the value of customers and memberships that lead up to the multi million dollar sponsorships that you see. Think about sponsorships its not just straight line projections from Man Us or Pool's - there are quality measures that go into it that will affect how much sponsors will pay. A declining core membership base is always worrisome and levy will be watching the "quality" of his season ticket revenue closely (loyal customers = higher quality in general).
You can read about how Netflix/Facebook stock valuations are impacted by its core or loyal customers - e.g. losing core while growing sends out mixed signals. What sponsors ideally want is adding to a loyal bunch of core customers - there's an assumption that new customers will be more sticky. Some churn is good but only against bad customers (e.g. regularly defaulting/ causing customer services issues). Generally churn isn't perceived as a good thing for any membership body.
I completely understand customer KPI's but I think you are fishing here
- In a Covid affected world, customer churn would be likely perceived as economic more than anything else
- Football clubs can't be measured like a subscription service or a Facebook because the product (actual stadium seats) is limited (subscription services have to show growth even more than churn).
- If our story is 40K season ticket holders with an 80K waiting list (making up numbers purely as example) and a 10% churn (I'd be amazed if season ticket churn is that high), I would not see how any advertiser would call that as a problem.