That complain in isolation is meaningless. People always complain about prices. If you ask me, I’ll tell you that Subarban is too expensive. Does it mean that building a cheap Subarban a good idea? No - even if given away for free I may still not buy it because the cost of insurance, maintenance and gas may be still too much for me, and I still go with Kia Soul instead. You see, I’m not a good prospect for Subarban to begin with, as your complainers probably are not good prospects for integration application.
People who complain that Zappier is too expensive may not be good customers for your proposed integration platform, too. Yeah, you’d get a lot of freeloaders who will flood you with feature requests and complains - but provide no value.
Think of it - if they can’t pay for Zappier, why would they pay for anything advertised on your platform? “It is too expensive!” Hence, why would vendors advertise on your platform? They won’t - the ROI would be next to zero. And here you left holding the bag.
Besides, the whole “advertising as a revenue stream” idea is dead for at least 10 years. Ads can cover some of your costs, but they cannot cover all.
Trello is not free. It is freemium, but very much priced for business and enterprise. Slack is also freemium and is very expensive for business and enterprise - my client recently had to abruptly halt its adoption when the price tag reached CAD 1 mln.
Free software is generally crap. A few exceptions only confirm the general rule. It is crap because supporting, maintaining and developing software takes time, time and time, and time in development world = money. When a developer of a free software decides should they fix a critical bug or complete a client’s project so their kids have food on the table - they choose food.