I have gone through all the questions on the thread regarding Coin charges and have concluded the exact point as you have highlighted.
In short lets say I make an investment of 10000 p.m. in ELSS for only two years and stop my SIP after that then I will have to wait for next three years of my last SIP amd keep paying Coin Rs.59 p.m. even if I am not using the platform anymore i.e an amount of Rs. 3600. In essence with 10% average return after five years I will get Rs. 24000 - 3600 as the net profit. Above this I found there are some charges to dematerialize the MF as well.
Why not use CAMS then? Also a new platform called Clearfunds is offering a flat Rs. 199 one time subscription charge for direct MFs.