You are missing one point, ARMs typically have a maximum increase and maximum total interest rate. Your max is likely to be 5% - 6% more than your initial rate, while that would suck, it isn't going to 20% like many would have you believe and relative to the late 70s, early 80s - counts for something. Those terms vary widely and are to be shopped and compared relative to what is important to the borrower.
I see a lot of benefit in 1% lower interest rate if the odds of A) Being in the house more than the ARM term are low and/or B) you are paying substantially more than the minimum payment. That is $1000 per year of interest that can be applied to principle for every $100,000 of loan value.
I also see value in flexibility. HELOC and lower required loan payment provide that. If you don't have the discipline to use a HELOC appropriately, what makes you think you will manage cash any better? In effect, a $0 balance HELOC can act as your emergency fund while that extra cash is put to work paying down debt, i.e. 4.5% mortgage versus 0.5% in some credit union savings account. That would be $400 per year of interest on a $10,000 account.
But yes, if you are completely undisciplined and a slave to temptation you should do the 15 year loan and hope you and your local housing market don't come on hard times. I would rather see folks (in this example) put $1400 more to work for themselves per year than the banks through some basic financial engineering. Compounding those savings over a 5, 7 or 10 year ARM pretty much covers any contingency for rising interest rates if you are paying the savings into principle. That $1400 of savings over 7 years is $9800 - 10% of the loan value. Add at least $1000 for the value of compounded interest.
30-Year Fixed Mortgage Loan Or An Adjustable Rate Mortgage (ARM)? | Financial Samurai