Yes, the opportunity to pay any fees by Debit Card would work extremely well, with a MF default option to collect fees due directly from the ISA if the Debit Card route did not work for any reason . All funds that the investor wished could then be kept within the ISA tax wrapper, if they were enabled to pay fees separately.
Although, the fees are very competitive and reasonable - if as an example an investor had £100,000 in an ISA, and a total of 2% was due in fees, the £2,000 would amount to 10% of the current ISA yearly allowance. There are a lot of ISA investors in the UK that have built up funds in excess of £100,000 and the Debit card solution for payment of fees would work well for many of them, potentially significantly boosting compounding.
Also I could see the Debit Card solution for fees alleviating a bizarre “mirage” type effect on the funds paid/fees paid into the MF ISA. To attempt to explain what I mean: if say an investor is obtaining an ISA return with a competitor that does not make him/her happy and it totals funds of £80,000, but it has low fees (say a self-select ISA) and they then switch the whole £80,000 to a MarketsFlow ISA, which gives a 5% better return overall.
Then the investor pays in his/her full £20,000 ISA Allowance to the MF ISA, there may be an inclination to feel that the £20,000 additional funds paid in becomes the bearer and focus of the overall additional fees charged by MF, rather than the investor seeing the fees being proportionately borne by the now £100,000 investment in MF. This effect would be completely deflected if the fees were paid separately by Debit Card.