Hello! While I grew my novel mainly using the sub for sub method,
I think it's fair to say that it can be difficult for the countless small creators on Tapas to grow their novels without it's help, since subs will circulate around the community.
What I do think is that there should be a limit on how many subs you have before you can use sub 4 sub - e.g. If you pass 200 or 250, or maybe even 150 subs, your novel has reached out to a number of people equivalent to a year group in a large school, and so you shouldn't need sub 4 sub anymore.
However, this method also makes it seem as though you lack enough writing ability to be able to grow your series by yourself - using sub 4 sub feels like forcefully gaining fake fans. But, the counter to this is simple - if you don't want to appear that way, don't use it, I suppose.
That's my humble opinion, thank you very much!