The second loan is not a short-term balloon note. Instead, it is a loan with a term of at least 15 years. With a portable mortgage, there are no points, application fees, processing delays, qualification hurdles, or loan origination expenses. Good debt is merely moved. Buyers save dollars and sellers will have cause to favor purchasers with portable mortgages because loan hassles can be avoided.
Portable financing challenges the economics which lenders use today.
That said, the loan business is a Darwinian contest where only the strong survive. We want to buy a replacement home and rent the one where we now live. A Buying a new home and renting the current one is the way many investors build real estate portfolios. Local brokers will be able to provide information regarding rental rates for like houses and an analysis of market demand. That is, how much down.