The downside could be something that nobody has imagined yet. That is the problem with change. I’m not against this, but I demand reasonable study. (but not unreasonable levels - vaccines and GMO have been studied enough to conclude they are generally safe despite people yelling more study needed)
Often car companies offer a rebate or the low rate. If you do the math the rebate is often for more than the difference in interest payments between the low and normal rate.