It stifles innovation and rewards big business. Why risk developing something if it will be taken from you? A winner-take-all kind of situation. It leaves little room for the platform's authors to generate revenue other then hiring themselves to said big business, and probably at depressed rates.

Because you can always outcompete them?

No, it is not a first class ticket to unicorn valuation, but it is trivially easy to differentiate

- from Google by just providing any kind of customer service to all paying customers

- from AWS and GCP just by allowing on premise solutions

- from Facebook by not reliably failing to protect and/or actively exploit your customers all the time

- etc

Need proof? See Slack, WhatsApp and Instagram (before the acquisition), Basecamp etc etc.