There are so many corporations that made similar pledges only to break them when it became advantageous for them to do so. Is there any mechanism that actually would enforce it? Mechanism that wouldn't disappear with a single stroke of pen?

If you keep controlling equity stakes in the business and don't need to raise significant outside investment it will work out as per your business model.

If you get into enterprise selling, you'll need a sales workforce and significant cost of Sales/Goods/Administration. If you want only organic growth, you'll be ok. But what will happen is that investment from, or selling to a large corporate, will turbo boost such sales; the temptation proves irresistible to many. If you are in a land-grab dynamics with other companies, growth may actually mean survival.

There is a scalability tier above the best subscription businesses, which is mass-market consumer adoption. The twitter experience demonstrates this. The ad revenue across billions outweighs a tidy subscription revenue from a small highly engaged subscription crowd. Any significant co-investor will find the lure of this irresistible. If you gain private equity investors, you will become ad based with high probability.

I welcome your contribution to the field of messaging apps and wish you well. I hope that this can inform your judgements about business models going forwards. If you think this is incorrect, please follow up with your alternative thoughts.