Personally, I'm glad HIStory was a double album.
To me, it felt like he was reminding everyone of why they loved him in the first place (after the worst allegations anyone could face), as well as the concept of 'past and present'. In addition, if it wasn't for the Greatest Hits disc, I don't think I'd be sitting here today talking about MJ.
The reason I got into MJ was because I saw Thriller on YouTube a good 7 or so ago. I played the song over and over and suddenly remembered Mum and Dad had an MJ album! So I rushed over to their CD collection, found the disc and studied the back. Almost gave up until I finally saw '9. THRILLER'. I ended up playing that disc loudly and discovering many songs I love (Billie Jean, Black or White, Bad to name a few) were songs made by MJ. If Thriller wasn't on HIStory, I would've likely not bothered with the disc and eventually moved onto something else.
Whilst lacking some of his best songs (notably SC), the Greatest Hits disc did not offset anything on the Present disc for me. I did listen to Disc 1 mostly at first, but I also ended up eventually listening to Disc 2 and discovering many more songs I liked. Obviously my views are largely influenced by my personal experiences but still, I'm far from the only person who's obsession with MJ begun with HIStory's Greatest Hits disc.
The double-disc set put the price up and probably did throw off the odd casual listener but admittedly, I don't really care about the price. It's not like it offset the albums sales too much, having sold over 20,000,000 copies (and wasn't that figure from before MJ's death? I've seen it float around for aaggeesss).
I agree about how there's too many 'greatest hits' discs for MJ, I'm seriously going to be annoyed if the Estate announces another one in the next decade or so (although in 20 years or something? I'll be totally down for that - introduce a new generation to MJ. Think what 1 did for The Beatles back in 2000).