I wouldn't turn it down, but I also wouldn't put the bulk of my money there. When the market historically averages over 8% in the long term, it's -EV to put it there just because it's safe. Now, if I were 5 yrs away from retirement, then yeah, I'd probably switch a higher % into that kind of account. But with almost 20 yrs away from that point, I'll put my money where it's very likely going to make significantly more.
But that's still different than a fixed mortgage payment. By the time you hit 30 yrs, inflation will far surpass that fixed 3.5% rate you're paying. Different animal than compounded returns that you're asking about above.