You can want seniors all you like but if your budget can't handle it you get juniors. This is how you end up with something that works bit doesn't scale well. By the time you need to scale you should be able to afford seniors.

I think the part about the budget was accurate but it might have also just been difficult to attract top talent without the name recognition.

I have my opinions which I will not share for why Hipmunk stagnated but I will say, it had nothing to do with scaling or technical obstacles.

Start-ups hire "juniors" but they don't advertise the position as junior, you can identify these roles by virtue of them paying way under "market-rate". The downside is most companies (it seems Hipmunk was an exception) have little to no training for these "juniors" and you have to have a strong stomach for BS as you have to make do entirely by yourself. YMMV.

I've been in a junior start-up role twice very early in my career. It worked out for me, but I wouldn't really advise it in an era of Google/MS hiring massive amounts of fresh grads with high comp.