I think this is a nice round number and something cool to aim for.
I also think how you get there shouldn't be restricted in any way (ie. no more raiding/black market/trading caps.), if you want to get there by raiding a billion UUs, or buying them, or trading for them, or any combination thereof, it should be up to you.
I also think that the idea of a plague breaking out once the limit is reached isn't ideal. I would want to feel safe acquiring UUs all the way up to the maximum without having to fear losing a lot if I accidentally hit the limit and always having to be careful of keeping it just under the limit.
I think maybe a better idea would be to justify the maximum by saying something like 'the resources of your home planet cannot support a higher population'.
And therefor your natural daily UP just stops working (I think this would lead to players growing their population to around 990m so that their UP keeps functioning, and just selling 10m UUs at a time whenever they hit the maximum).
What do you guys think?
(Note: if your total population size is greater than 786,164,384 disease will surely strike your lands.
Above 589,623,288 total population, raiding and black market purchases are no longer possible.
Above 393,082,192 total population, trading for units is no longer possible.
Pergatory is not accessible above 58,962,329, and will forcefully eject you above 78,616,438
Current trades count towards these limits.)