"There's no point launching an app ... all the good ideas have been taken"
Is that really true?
It depends on what you want.
You might not start the next unicorn. You might not become the next Elon Musk.
But there are always problems to solve.
Problems that people will pay good money to make disappear.
Problems that seem immense to your clients but are simple to you.
Choose an audience.
Do your research.
Find out where they get stuck.
Discover what they'll pay to get fixed.
Focus on solving it really well.
And then you'll succeed.