You're mixing up two things
You're mixing failure to (properly) produce what was promised with obsolescence. This isn't crap because 30+ year old computer designs are crap by today's standards. This is crap because the execution is completely botched. It freezes (the old Speccy didn't do much of that), it arrives with visible physical damage, the controls are bloody awful (you have to press the directional buttons like crazy) and ~98% of games are missing. There is no need why any of that would be true (actually, this ought to be quite easy to not mess it up).
As for the other thing, it may be that, even if it had been done properly, people would have had a bit of fun at the beginning and then put it away forever, but in the age in which Flappy Bird was a smash hit not long ago, there's a plethora of Spectrum games that one could get hooked on.