I have a lot of clients using MailChimp's free plan which has worked well for them. I am grandfathered into using all Google's services for my business for free (ignore usual criticisms of Google) because I got in early.
I've worked in this industry for over 20 years and found that the more complicated your explanation, the less likely clients are to want to work with you. Being a stick in the mud on usability is another example - they'll leave for someone who'll make them a splash page without question, etc.
I certainly understand that. And grandfathered or not your use of Google's services for 'free' for your business is coming to an end.
As long as you understand the risk you are carrying you can price out the externalities of it going south.
To explain that, consider this artificial example made up based on what you just wrote; "I have a lot of clients using MailChimp's free plan ..."
Now lets say MailChimp has a tough few quarters, maybe they get a new CEO or maybe they just need to get more money for their services than they currently do and so they revise their "free" plans into non-free plans in ways that prevent some chunk of your customers from using their free plans.
Your customers are forced to change their previously working system for a different system. Four outcomes pop out of that event; They can use their current system but now it costs them more money, they can hire you (more money) to come up with another free system which they now know will only last for some random period of time, you can design a new system for them for free (costs you money), or they can go with a new contractor who will design a new system for them.
As the original provider of the system, you cannot predict when this event will occur. It may occur at a slow time, it may occur on black friday when it the customer's busiest time of year. And as the original provider you cannot predict the new cost of the system, or the cost of a replacement system.
So the risk you are carrying here is mostly reputational (some of your customers will say bad things about you if this event happens at a bad time or costs them a lot of money they were not expecting to pay.) Depending on what type of warranty you provide or imply for your services (most people disclaim these so the common case would be to leave the customer with the costs) you may be carrying some financial risk to correct the future situation.
I will reiterate that there is absolutely nothing wrong with carrying this risk, as long as you are aware you are carrying it. Because you can plan for the various cases (for example by continually researching equivalents to MailChimp so that in the event this happens you can quickly and easily move your customers over to the new solution)
I've worked in this industry for over 20 years and found that the more complicated your explanation, the less likely clients are to want to work with you. Being a stick in the mud on usability is another example - they'll leave for someone who'll make them a splash page without question, etc.