Keep rentals mortgaged or primary house?

[Tj M.]

I am having a custom home built and when I sell my primary residence I will be able to pretty much pay off the new house. I also have to 2 mortgaged investment properties one is $90k at 5.8% interest and other is $200k at 5.35%. I want to mortgage the new house at like 4.1% interest and pay off the rentals which I calculate would save me about $700 a month. Any reason why I shouldn’t besides dead equity/loss of opportunity cost?