The problem is that most console gamers newer ever buy upgrades. So there is no demand to make games that need that upgrade. That means that there will not be games that needs those upgrades, so essentially allmost nobody buys them...
In PC world upgrades are no braners, but in console world have not been very big succes (not counting game controllers).
So upgrade can be succesfull, if there is no any need to support it any way. SSD disk would be one, to subtitute the hard disk. It is faster, but does not need any kind of support from the programs.
PC-consele would have it. All PC games are made for different configurations, so a console type PC with game only UI would make it, but... Nobody is willing to sell just a machine at discount (like consoles are). Why make only gaming machine, when it can normal things like web-surfing, word prosessing etc.
Summasummarum... Upgrade able parts are good (the PC systems), but gaming consoles are not so good platform for it. There is very easily an situation where you have super good upgrade that nobody is willing to use...