Totally. See I don't doubt 390X will be faster, even much faster. With 8gb, it will be viable for decently long, so I can justify the price. But if it's only 4 gb, my dilemma is, do I want to pay ~3x what the 290X costs for something that I may need to replace in less than 2 years.
What makes you think there won't be in-between options like $399-499 R9 390?
I think a lot of gamers are scared about high prices because NV's marketing set the stage that makes many believe $500-550 is now somehow the status quo/normal for mid-range chips. It isn't and AMD will prove it once more with R9 300 series. Remember HD5850 was $259, HD6950 was $299, HD7950 was $449, R9 290 was $399. Even if Fiji XT is $500+, there will undoubtedly be a 2nd tier R9 390 card below $500.
Also, you'll still have the option of buying a 970 with TW3 and Batman come mid-June. I think right now $280 for an R9 290X is too expensive since GTX970 is $315 with $60 worth of games that can be readily resold or used if you are interested in them. Alternatively, I see PowerColor R9 290 for
$220 on Newegg. If you just want a stop-gap 15-18 months solution and don't want to spend $500+ on a 4GB card to keep beyond 2 years, why even pay $60 extra for an R9 290X over the 290? You could grab this R9 290, sell it for $100-130 in 15-18 months and just ride the wave all the way to next gen cards. I honestly don't see how 290X is worth $280 over a $220 290.