It's gone from being the #1 sport in America to barely registering on the sporting map on pages like ESPN. I think it's fair to say it has, but oh well.
You're talking about popularity now, not the standard of the practitioners. Boxing is in decline as a specticle (though attendances in the UK are on the rise, and last year boxing generated more money than ever). It's certainly less popular, but worse? I don't see it that way, I think the last fifteen years is comparable to any fifteen year span in boxing in terms of quality at the top, and I think that the last 8 years isn't hugely inferior to any 8 year period in terms of quality of the very best involved.
Less popularity leads to lesser following which leads to lesser participation and a smaller talent pool to draw from, which is why I'd say it has declined in that sense, the quality and quantity of top fighters are not what they once were. I think if you compared this generation to say, the 40's, there'd be a very marked difference. Jones is the only active fighter(past prime or not) that I can easily put inside my top 50 of all time.
Well, I don't know that there are a LOT of years where you can "easily" place a fighter in your top 50 (108 years of boxing v 50 slots). Jones is the right now addition and Hopkins is an arguable conclusion (though he just misses out for me).