Is it safe to assume that Bitcoin is not soft-fork backward compatible?
The only benefit of a soft fork over say a hard fork is that old nodes aren’t forked off the network if they fail to validate all the consensus rules. In the case of a hard fork (a relaxation of the consensus rules) an old node could reject a valid block and then be stuck forever because other blocks would be mined on top of that block.
What are the technical risks to the security of individual pre-Segwit node runner?
If you don’t check all the consensus rules (including all soft fork rules that have been activated) then you risk accepting an invalid transaction or invalid block. If the majority of other full nodes and other miners are enforcing these rules it shouldn’t be a big deal. You’ll end up later following the chain with most proof-of-work (PoW) and discarding the block you initially thought was valid but turned out to be invalid. But you are in a stronger position if you don’t rely on others and validate all the consensus rules yourself.
What are the detrimental effects to Bitcoin as a whole if hypothetically majority nodes switched back to non-Segwit version of nodes?
If the majority of full nodes and miners didn’t enforce all the consensus rules there is a much higher risk of an invalid block being accepted by the network for a longer period of time resulting in larger re-orgs and more confusion on the latest state of the blockchain. In an extreme case miners could continue mining on top of an invalid block forever and then it wouldn’t be clear what consensus rules were being enforced and which weren’t. The entire system breaks down if full nodes and miners aren’t sure what the consensus rules are and which consensus rules should be enforced. If a few full nodes don’t want to enforce all the rules it isn’t a big problem. If the entire network doesn’t want to enforce all the rules it starts to become a much bigger problem.
