Let's assume Scirra are converting 8% of their 100,000 users, that's an income of £608,861.88 a year if we use an average of $99 a user, but we'll call it £610,000 for easy maths.
Lose 2% (£12,200) for transaction handling - £598,000
Next up, staff salaries; let's assume a team of 6, earning the average London wage (£37,000); all together that'll cost £222,000 - you've now got £376,000.
Don't forget NIC - that'll be £4k per person, or £24,000; now you're on £352,000.
Better get some offices; average for London is £650-1,5000 per person; and Scirra have nice serviced offices. Let's call it £700 per person, it works out £50,400 a year. £300,000 remaining.
You'll probably want some tech for those offices. Some computers, monitors, chairs to sit on. How many devices does Construct work on? All of them? Crap, you'll probably have to buy some phones and tablets for testing purposes, how much does the new iPhone cost? £1,000? Let's call it £10k all in. £290,000 left.
Your product is web based right? You'll need to host it, and the multiplayer signalling server, oh and the build server - don't forget about the website and the forums either. It'll need to handle lots of simultaneous connections from all over the world, not to mention have a really high number of fallbacks. Let's fluff the numbers here and call it £15,000 a year in hosting. £275,000 in your bank.
You'll probably want to drum up some marketing now, in-house talent is all good, but you'll likely need an agency for stuff like exhibitions, print work, PPC and outreach. £75k will get you a decent agency for 40 hours a month and give you some buffer for budget and outcosts. £200,000.
All done? Nope. Tax! 19% (£38,000) please. You're down to £162,000. Seems like a lot? Using our numbers above it costs £37,333 a month to run the company, so what you're left with is just over 4 months of runway.
Sure, you could hire a new person, but then you have to factor in the extra salary, the cost of recruitment, and the time spent on-boarding. This is excluding any kind of interest repayments you might have with investors or a bank, or dividends for directors and shareholders. Plus loads of stuff I forgot!
The above numbers are entirely speculative, but it's unreasonable to think that Scirra are actively choosing to not scale.