I'm looking into the future on this one. But, I want to make sure my client's contact forms will keep working in the future if for some reason I do not still have a paid Webflow account.
It will as described in the free plan: 10 entries/month.
I'm not good at forms, but you're not obliged to use the webflow form engine for the forms of your webflow sites, wether they are self hosted or hosted by webflow. For example, if you have a Mailchimp mailing list and a form to subscribe to it, you can use webflow form component to build your Mailchimp subsc. form. It will call directly Mailchimp servers and not Webflows'.
And I guess there are tons of sites to build form with.