I'd recommend being very careful to keep licensing and work separate - both in terms of invoice (separating each line item) and paperwork. This helps prevent confusion on the part of the client when they, inevitably, miss remember the agreement and innocently assume the new thing they are asking for is already covered.
In that case the question you asked would be a licensing add-on that would not effect a work contract.
If the client were to request a change to the scope of work (say they add a style to the deliverable) then that does affect the contract. In most cases where a contract is in place and you are changing a detail you'd do an addendum to reflect the change, not a whole new contract.
One, pricing for add-on licensing is rarely done in a vacuum. Foundries price web embedding, just to take one, based on as assessment of the use (usually traffic) as applied to pricing tiers. So, the only choice when pricing for "unlimited clients and unlimited use with no time frame" is to price at your highest possible prices for a very high number of clients and uses. And even then you might be wrong.
Two, unless they are required to report each client project to you then you how no way knowing if a given observed use is licensed. And I'm sure they don't want to report to you since, as Pablo pointed out, the whole point of this seems to be to remove all friction for them.
If I did agree to this I'd do it for a short period (one or two years) as a test. I'd require they keep careful records so we could review and reassess the pricing at the end of that term.
As a business side person I need to build my schedule around client need. Yes, I have my own projects and I'm involved on internal projects but the key scheduling factor is when I will be needed for technical, sales and licensing support by clients.
Our clients are all over the world (as is true for most foundries) with only about 45 percent in the US and about half of that 3 time zones away from me. It works out that even though I don't need to work more than 8 hours in a day I need to be available from 7am EST (at the latest) till 8 or 9pm EST. There are gaps in the middle of the day when I'm less needed and a few key times right before various key time zones leave for the day that I need to be especially reachable. I tried to work from an office and it just didn't make sense. I ended up either spending more time there than is healthy or doing most of my work in off hours and just lounging in the office. Working from home allows me to work organic hours as needed and still have a life.
You know me, I'm all about simplicity of message. As the initial message, we tell the client that there's a fixed price per style but that we can add discounts if they request a lot of styles. As the process goes on we are essentially pricing by the master and adding a little for the interpolated styles. But if you start by trying to explain this really complex calculation then I find that you loose most people.
we've seen a decrease in the same period (not as big but still noticeable). I had thought it was because our biggest seller has been around a while and was losing steam but perhaps it's simply industry wide? Are other indie foundries seeing the same?