I think that you will find many people will say yes if asked if they leverage current low interest debt such as a mortgage to invest but when asked if they would accept new low interest debt at the same rate in order to invest then the answer changes to no.
This does not make logical sense
I think people can be scared of the "liability" a debt makes because you MUST make the payment EVERY month creating a constant required drain on your account.
-Could the payments for a 30 year $1 million loan at 4% be sustained with a $1 million investment?
In all likelihood yes and you would be better off if you did it.
-Would I personally accept a loan like this in order to invest?
Maybe
To me it has to do with what would happen in a catastrophic loss situation. If you try to leverage a sum of money many times larger than your income/networth/investments than there is a very small change you could have your finances catastrophically devastated.
I choose to leverage a 3.875% mortgage right now and if I had an opportunity to leverage further debt at that rate I would... but I would not try to leverage millions of dollars like you suggested in the original post unless my income/net worth was much higher.
One problem is where do I get a loan with minimal setup fees at a low interest rate? The banks need a reason to lend you that type of money at that low interest rate and I don't think that reason will be "I want to invest it".
I think a good balance of risk for people (with self-discipline to invest the difference) is to not be scared of debt and to leverage their current low interest debt for profit but not seek further debt for the sole purpose of investment
Also if you haven't ready MMM view on this type of quesiton:
http://www.mrmoneymustache.com/2012/04/25/unlocking-your-home-equity-for-profitable-investments/http://www.mrmoneymustache.com/2012/02/24/pay-down-the-mortgage-or-invest-more-a-winwin-question/