I don't think it would hold up to modern shows, if it was airing for the first time today. Firstly because BtVS is very much a show of the 90s. It speaks volumes about life in the late 90s and what it was like to be a teenager in the late 90s. Teenagers today face similar issues, but in a different way. Technology is at the forefront of everyone's lives. Buffy didn't even get a mobile phone til S5/6!
Modern shows, whilst they look good and have the budget behind them to do so, fail on the characterisations and emotions that BtVS did so well. But for the vast majority of TV viewers nowadays, how a show looks (or the starring roles) are more important than great storytelling.
TV has proven that period shows do work (Mad Men et al) but the difference being they are modern shows with a modern budget showing a set period of time, rather than a 90s show with a 90s budget showing that specific period of time. That's why stuff continually gets rebooted (usually to the detriment of the reboot), because people would rather see a 'new' version that's substandard, than an 'old' version that looks old.
Also, a lot of the love that the show has is not only for the great characters and writing, but also the nostalgia-factor. I love Buffy because I remember growing up in the same era, with the same dodgy fashion.
Sadly, I truly believe that if Buffy were airing S1 now, it would have been cancelled after 12 episodes. It would never find its feet and run with it.