Well first of all I don't care about album sales - they don't dictate music quality. But if someone says 50 has fallen off because he's not selling as many records as before, you might as well say Nas has fallen off too because his albums aren't selling as much anymore either. God's Son took a year to go platinum. Street's Disciple took a year to go platinum. HHID sold 300,000 in its first week. Meanwhile LIG has sold 300,000 in a year. 50 seems to be having similar record sales as Nas recently - so if Nas is relevant, how isn't 50?
Also I don't agree with your argument that he can't create a buzz with those names. Who is going to get excited and want to buy a 50 album because of Adam Levine or Alicia Keys? I wouldn't even want to buy a 50 album because of Eminem because he has put garbage ass verses on all of 50's albums since Massacre. Plus guest appearances don't help sell record imo - you buy records to listen to the main artist, not their features. The D12 albums didn't sell that much in comparison to Eminem's main albums for example, despite having Em on nearly every track and coming out after MMLP and TES.
Nas doesn't appear to be doing the same numbers as you indicated in the bold, his last 2 albums appear to have dropped off at the same rate as 50's. And honestly I don't care what other people rate albums as, LIG is pretty boring and doesn't have the street sound I expect/want from Nas, while BISD has the sound that I expect from 50 and is full of bangers. Like I said before I'd take BISD over Nas' last 2 albums, so I don't agree that 50 isn't "relevant music wise" - especially if you compare his current work to his previous work (it's not like you should expect his music to change over time imo - and his style was never the kind to get recognized by Harvard or w/e bs).
50 Cent
GRODT (2003) - 6x Plat
The Massacre (2005) - 5x Plat
Curtis (2007) - 1.3 mil
BISD (2009) - Gold
Nas
Stillmatic (2001) - 2x Plat
God's Son (2002) - 1x Plat
Street's Disciple (2004) - 1x Plat
HHID (2006) - 1x Plat
Untilted (2008) - Gold
LIG (2012) - 372,000