Before 1991, it would have probably been "A Rolodex can only hold 10,000 cards" for most people. The solution would be to buy another Rolodex.
In real physical rolodex you can understand the limit, because it really exists. In Gmail it's hardcoded "OK <= 10000" and that's hard to understand, specially when there's upgrade for 20,000 contacts - so 10,000 is just an artificial dumb rule.