Keep in mind that cryonics generally advertises the possibility of in-place repair of some kind, biological revival. The uploading possibilities are not optimized for. The kind of compounds you would want to add to preserve information (to avoid loss due to denaturation) are very toxic at much lower concentrations.
With regards to plastination, it has the extra destructive steps of trying to get a solid in the end, and to avoid cutting it into pieces.
ie the typical opinion of Very Serious People
I doubt it's the typical opinion, really. If by very serious people you mean top scientists and the like... they have more complex opinions because due to the training and intelligence they can relatively effortlessly hold complicated relations in the heads. Opinions could be "no future technology will permit revival of [currently] frozen corpses", "freezing and biological revival is unlikely to ever be workable", and so on.
And on a tangent, correct opinions about such topics are a matter of knowledge and capability involved in simulating said processes in your head.
To deal with a simpler example without cryoprotectants (e.g. as described here). When a scientist with relevant expertise considers dropping a head into liquid nitrogen, within mere seconds they do a lot of work in their heads - they correctly estimate the cooling rate inside the head (going to take many minutes to freeze), they picture the ice crystals growing, everything other than pure water (up until -18 celsius) getting squished into inter-crystal spaces and getting ripped and scraped in the process (irreversibly losing a lot of information due to many to one transitions - and no redundancy will help you when the 'redundant' storage is subject to same destruction), chemical damage due to high salinity (also irreversibly losing information), and so on and so forth.
The rationalists on the other hand seem not to even realize that such considerations are required, let alone occur. It does not matter for how long you are going to think about it if your thought is not even simulating any destructive processes that occur.
With cryoprotectants, the issue is considerably more complicated, with lesser possibility for a simple conclusive disproof, but no better reason to expect it to work (pumping brain full of solvents at denaturing concentrations doesn't seem like a good idea, and all those references to rabbit kidneys at much lower concentrations mostly serve as evidence of bad faith rather than evidence that it works). The one perhaps big advantage though is that at least the connectivity map would be readable for sure, that's provided that the cryo-protectants actually do reach most of the brain, which is uncertain as well.
It's clear at this point that your opinion is not as extreme as I the impression I originally got (unlike some people) and I don't really disagree much with what you say here. I too am skeptical of current methods (and I'm not signed up for this and other reasons), but I'd like to see further work on both traditional cyropreservation and other methods such as plastination, taking into account any new research on memory formation and storage. The idea being to get to a point where we can preserve an animal brain and check to see that the important information seems to be preserved (even if we can't read it back out yet).
In the past, the Cryonics Institute has had a policy that said that they would not accept anyone who is not a member. This has changed. The policy now is that someone who has full legal authority over your body can sign you up after you die. It costs $36,250 to be frozen if you are not signed up, which is more expensive. They also will not do anything until you have been on dry ice for 2 weeks after they have been contacted, so not being a member is more risky.
This is very important news for anyone who is currently cryocrastinating. It means that you can drastically increase your chances of survival without filling out any forms. All you have to do is tell a loved one you want to be frozen upon death, and that you would like them to take responsibility for making sure this happens. This takes literally 30 seconds. Do it now!
This news might also be a reason to not sign up right away, if you think something better (like radical life extension or uploading) will come along in your lifetime. We should discuss this in the comments.
Edit: The general consensus of this discussion is that this is a really bad reason not to sign up for cryonics.