Popular Istagram accoutns have such a short shelf life, beacuse thet tend to go like this:
Person is passionate about something
Person starts Instagram account to give tips (midwifery, cleaning etc)
Person becomes popular
Person thinks: great, I can give up my day job and earn money from my Instagram
Person starts to spam followers with photos of weird shit they've been 'gifted' or of their perfect life
Followers leave in droves because no one, literally no one, is bothered about seeing gifts some random person has been sent
Person goes back to the day job.
The only exception I can think to this is the Unmumsy Mum, who I follow because I'm renovating a house. But I might unfollow, as her House Project account has started to have a few gifted posts (always of shit things!)