That's the idea, except that it is being done with an eye on the element of risk, which is so often forgotten by folks that want to play on-the-margin schemes. Paying extra on the mortgage is an extremely safe, guaranteed return on that investment (if that turns out not to be the case, then we have much bigger issues to deal with). Taking the same extra amount and investing it in an unsecured investment that will probably return substantially more but might lose value has to have a substantially higher likely return to offset that risk. Given that we are talking about something that has a major impact on our lives in retirement (which is pretty imminent for me), my tolerance for risk on this one is pretty much non-existent. Hence, I'm willing to let the funds set in an insured money-market account that returns more (right now it is paying 5.12% APR) and, if rates start going down, I might put it into as long a term CD as makes sense given the rate structure at that time.As long as interest rates stay high, pay the minimum and invest the difference.
Another factor in this is that I hit the 59.5 year age point in less than a month, at which point I can withdraw funds from my retirement accounts. So one option would be to pull enough funds out (taking tax issues into consideration) and augmenting the money market so that, as long as we pay the escrow amount into it each month, it will be adequate to pay off the mortgage in time (and that will have to be revisited as interest rates change). I can probably do something similar within the retirement accounts by finding a suitably secure investment. The money market that is attached to the account has basically zero return and is only useful as a holding account for cash between investment actions.