Another group of customers could be people drawing down on investments or pensions in lump sums and then spending that money. It might not be loads of people but they still count.
I think monzo should look at 90 days instead of 35.
Also some older folk make far fewer transactions than working people.
I completely get what you are saying. especially the large amount of savings factor and flex card usage. an easy improvement could simply be if above £5k in savings the income every 35 days is no longer important since savings also is one of the main benefits of a bank (storing your savings in, the more money you have the better for banks), and then if spending on a regular basis that counting also instead of needing two direct debits. completely get it, I use Monzo for everything too, business account, savings etc.