Well, that's the point, isn't it? The best thing Apple had on the table in 1990 is arguably substantially inferior to the 1988 incarnation of the NeXT hardware. (Also of note is the fact that the 68040 versions of the NeXT computers hit the market almost a year earlier than the Quadra 700/900.) A more "typical" Mac of the era was, again, something like an LC or IIci. (And your average hard drive was something like 40-120MB; the original NeXTStep 1.0-targeted hardware had a 256MB optical drive, and the system requirements for 1992's NeXTstep 486 3.3 recommended at least 330MB for a "stand alone" system.) The cost to upgrade an existing Mac to something like 16MB of RAM and a sufficiently large hard drive would have been somewhere in the $1,500 range in 1992 (the RAM, oddly enough, would be even more expensive in 1993 because of an industrywide shortage), and, again, they would have run the OS pretty badly.
(NeXT/OpenStep actually had a reputation for being somewhat sluggish even on the genuine NeXT hardware; part of the blame goes to the really ill-conceived idea of using the optical drive for the OS in the base configuration, but even with a hard disk it was simply a little bit ahead of its time in terms of system requirements.)
So, yeah, I just don't see it working. OS/2 2.x debuted in the 1992-ish timeframe and its "8MB to run well" system requirements were considered formidable for an OS aimed at the mass market. (Windows 3.0 would run mostly-okay with as little as one or two MB.) Mass market computers simply weren't ready for UNIX-class operating systems until around 1996 or so. (That's about the point that 16MB became "standard"; don't forget, the original iMac, circa late 1998, shipped with only 32MB.) Combine that with the fact that Apple had to suffer through most of 1993 and into 1994 with no systems that could really be legitimately called "workstation class" in terms of CPU power (IE, that uncomfortable gap between the March 1993 introduction of the Pentium CPU and the March 1994 introduction of the first three Power Macs) any attempt to wedge NeXTstep into the product line in 1993 would have been an unmitigated disaster.
(NeXT/OpenStep actually had a reputation for being somewhat sluggish even on the genuine NeXT hardware; part of the blame goes to the really ill-conceived idea of using the optical drive for the OS in the base configuration, but even with a hard disk it was simply a little bit ahead of its time in terms of system requirements.)
So, yeah, I just don't see it working. OS/2 2.x debuted in the 1992-ish timeframe and its "8MB to run well" system requirements were considered formidable for an OS aimed at the mass market. (Windows 3.0 would run mostly-okay with as little as one or two MB.) Mass market computers simply weren't ready for UNIX-class operating systems until around 1996 or so. (That's about the point that 16MB became "standard"; don't forget, the original iMac, circa late 1998, shipped with only 32MB.) Combine that with the fact that Apple had to suffer through most of 1993 and into 1994 with no systems that could really be legitimately called "workstation class" in terms of CPU power (IE, that uncomfortable gap between the March 1993 introduction of the Pentium CPU and the March 1994 introduction of the first three Power Macs) any attempt to wedge NeXTstep into the product line in 1993 would have been an unmitigated disaster.