I don't know when things started going downhill for Sensi, to be honest I've never bought a pack of Sensi Seeds apart from Mexican Sativa years ago and they didn't germ. Sensi were always too expensive compared to what else was available. The Geoden law was introduced in 1998 and that was when the general decline set in.
Serious Seeds saw a decline in some of their lines after 98 due to loss of parentals, the Kali Mist and AK47 changed and not for the better, some say White Russian isn't what it once was either.
I don't know if things declined at Sensi after Nevil left, the newer Sensi strains like Marley's Collie, American Dream etc are ones I know very little about, never seen em grown, in fact, you see very few Sensi Seeds grows online these days and it's been that way for years. I don't remember ever seeing grows of Silver Haze for example. I'm struggling to think of any Sensi grows I've seen last few years to be honest.