The Ridbaz (on the Rambam) says that this answer of the Gemara is difficult (Dachuk). I wonder what is the difficulty more that other Ukimtot in the Shas?
Can it be because we don't understand the Gemara before ("Kulle Ba'i Leshalume) as if it doesn't agree that these kinds of loans can preserve the Shemita somehow?
These type of loans definitely "preserve" Shemitah. L'halachah, even a loan for "Eser Shanim" preserves Shemitah.
I am not sure that the Radvaz was saying that this is more or less Dachuk than any other "Ukimtah," rather Ukimtaos in general are Dachuk, and the Rambam chose to avoid them by bringing the case as "Chamesh Shanim."