I would think ten years is plenty long. If those units use MOVs as their main mechanism I would go ahead and demote them. You say you spent a good amount of money.

Do these units have some sort of function indicators? If the indicator is telling you it is still okay, and you have a lot of faith in the brand and model, and that the indicator is telling you the truth, you could keep them in place and ride on.

There is no practical way of testing such units. So your operating on faith, company reputation, perceived risk, and how lucky you feel.

Then again ten years is a very long time in technology and consumer electronics. After ten years you might consider that you have got your moneys worth.

If you wanted to split the difference you could transfer the older units to less important equipment and bring in newer units to protect your most important electronics. There is some benefit to having surge arrestors on many circuits.