CGI, digital effects, along with everything you mentioned didn't exist. Taking this into consideration the visual effects in 2001 are absolutely stunning and ground breaking. The kicker is they have held up for 50 years!
Having grown up with Star Wars, the later Star Trek, BSG, Babylon 5, etc... SciFy in film and on TV they have become desensitized to the herculean effort it took to imagine and accomplish what Krubric put on film. Because today CGI is the norm.
It is also a film that, like Interstellar forces its audience to THINK. It doesn't spoon feed the story to the audience, and isn't full of mindless, non stop action or endless space battles containing lots of explosions or ever weirder and weider "aliens". Hence, some viewers, especially today in our low attention span society find it "boring".
The cool news is they are releasing a genuine 4K UHD HDR Blu-ray disk of 2001 taken from the digitally restored 70mm film. It is on my "must buy list". One more thing, because there is no CGI used in 2001, it will be a genuine 4K transfer, whereas CGI and digital effects in modern films are shot in 2K as a cost saving measure and the 2K is "upconverted" to faux 4K.
4k UHD MONOLITHS !!!