Beyond bugs, scaling your MVP to 1B users will mean expanding your userbase beyond English speaking Americans. This requires upgrades to internationalization, accessibility, possibly compliance with international laws and 3rd party licensing changes per region. Multilingual support staff and international payments processing. With a userbase this large, expect to be sued by people around the world, so you'll need region-specific legal services. Some of these issues just require money and non-technical staff and don't directly impact the user experience aside from diverting resources away from building features and fixing bugs for your original userbase.
Sure, but these aren't business model problems, they're business growth problems. The concern wasn't how to find 1B users in the world (and what do you have to do to get their money), it's whether scaling to 1B users inherently breaks the product, not just for individual users, but for all users.
If a company was only able to sell 2.6M copies of their digital software before running to expansion problems... good for them! That's a lot of sales and they probably made a great deal off of those sales. Sure, they can grow to 1B users, but they don't have to. There's no requirement for them to do that other than choosing to expand into those markets, and that's strictly optional. The business model is doing fine, there's no need to adopt a recurring payment system for ongoing maintenance.
And let's be honest, even if they do choose to expand into those other markets, the cost to convert the existing product to work in those markets is most likely less than the money they'll earn from selling in those markets, so... is there really a need for recurring payments to support maintenance? Will one-payment sale structures inherently fail to make the product profitable in a given market?
There are apparently 2B English speakers in the world, so you could in principle get away with no internationalization and have 1B users. The other things are more a cost of operating a multi-national business, and not a marginal cost of the software as such. You could also in principle scale to ~300M users (or ~100M households) without worrying about international issues by sticking to the US only.
You can tell the people who have never run a business or have worked at one small enough that they see everything. Support staff are not free. Project managers and salespeople can’t keep up with meetings and start sprouting assistants and coworkers. Customers are expensive, especially upset customers. So then the developers have to spend a lot more time making sure customers don’t get upset.
