No, you were not unreasonable at all,
Most of us will reach a point when we are no longer safe to drive. I went through this with both of my parents.
Unfortunately, the reaction to being told that we should no longer do things is usually to be hurt and therefore angry with the person who mentioned the matter.
I hope your husband realises that you are right and that you mentioned the matter because he asked and because you are genuinely concerned that he might cause or be involved in an accident.
The hard part is knowing where to go from here, isn't it?
Could you persuade him to have his eyes tested? Cataracts make seeing the difference between light and shade hard, and can restrict ones field of vision. As they can be easily removed today, it is stupid not to check whether he has a cataract forming in either eye, or even in both.
Glaucoma cannot be cured, but its progress can be delayed, and the sooner you get treatment the better the prognosis is.
Yet another good reason to make an appointment for an eye-test.
He might after all only need glasses to be a safe driver once again, (unlikely, but perhaps a selling point re the eye test.)
If he won't have his eyes tested, then I am afraid your only option is to refuse to be a passenger in the car.