That assumes consumers only care about capacity. I want single platter drives but I'll consider 2 platter or 3 platter drives if they don't make the single platter an option.
If you compare platter to platter (2 x 1TB @ 7200 vs 2 x whatever GB @5900 or @5400) you still get a noticeable power difference at the lower RPMs. And honestly I don't give a damn about the extra GB or TB.
I'm literally typing this on a system with a 2TB green drive that is setmaxadress limited to about half of that that is then partitioned into C and D drives and I'm only using about 150 GB out of the 2TB capacity. I'd be fine if this 3 platter HD just had less platters and was sold as a 1.3TB drive, a 1TB drive, or even the single platter variant of say 650GB (I'd rather it not be called 666GB but that's just me).
Really, it's my only drive in the system and I have 25GB on C and 125GB on D. I wouldn't be limited by a single 1 TB platter. I'd still short stroke it but I'd be happy to have the power savings, reduced noise, reduced heat, reduced vibration, greater shock resistance of reduced platters.
I know this is an older drive but I think the comparison is still valid in terms of the engineering trade offs of more platters
WD Black 640GB vs 1TB comparison
Drive Ready Time 11 sec 13
R/W Power watts 8.3 8.4
Idle Power watts 7.7 7.8
Standby watts 1 1
Max shock 300 250
Performance seek 29 33
Quiet seek 26 29
and that is a comparison of a 3 platter vs a 2 platter. The difference can be even more pronounced when going from 2 to 1.
So going full circle, sure 3 platters @ 7200 RPM is similar to 5 platters @ 5400/5900 RPM but there is room to go further. If they take green drives away and replace them with 1 and 2 platter 7200 RPM drives then I'll be all for it. I'll take the extra RPM at lower platters. If they take them away and force the purchase of 3 platter drives then I'm going to look elsewhere. I don't want lots of platters at high rpms.