First of all, you cannot borrow against a HDB leasehold, with the narrow exception of the HDB Lease Buyback Scheme. If you ever run out of cash due to cashflow problems, even by one dollar, then you must sell your HDB unit. Or try to rent out a room, but that may be awkward at best if your unit is fully occupied. Plonk down S$50K in accelerated repayment and you’re at least S$50K closer to a cashflow problem. (“At least” because that S$50K can earn interest.) Want to take that risk? The reason a mortgage borrower loses a home is because they cannot make a payment, not because they haven’t paid a mortgage faster. (HDB is a very forgiving, patient lender, though, that does everything possible to avoid that.)
Second, your payments are presently covered under the Home Protection Scheme. If one of the mortgage holders dies far too early, the HPS pays off that person’s share of the remaining mortgage. It’s a form of life insurance. Accelerate repayment of the mortgage then die too soon and you’ve effectively lost much or all of that S$50K. HPS would have taken care of it in the alternative.
Third, 2.6% money is still cheap money. A diligent saver and prudent, long-term investor should be able to do reliably better than that. One example: CPF Special Accounts are paying at least 4.0% interest, and 4.0% is definitely a bigger number than 2.6%.
Fourth, sub-2.0% mortgages are available from the private sector if you ever decide to refinance, i.e. even cheaper money is presently available.