As said above, there is very often a floor to tracker rates i.e. a rate below which they dont fall. This is in the T&C, although a few lenders have waived it recently.
Also, most people's mortgages are not interest only mortgages so, although the interest element will be very low, they will still have to repay the capital (the amount they borrowed) each month.
Buy to Let mortgages on the other hand are more often interest only (you would repay the capital at the end of the term, not monthly) and these payments can get very low as the interest rate tumbles - as I know from personal experience!