As someone old enough to have used search engines extensively myself and watching over others using it during the AV - Google transition, I can definitely say that defaulting to AND was one of the most important reasons Google appeared to give better results than AV for both advanced and basic folk.

Defaulting to AND means that you aren't just searching for the most common term in your query that drowns out the rest. Also, adding terms narrows the query down rather than making it more general. This behavior strikes me as far more natural for "normal" people.

sure, i agree, except that they also apply this 'narrowing down' logic when insufficient or no results are returned, thinking the query needs to be more specific in order to work. i have observed the following sort of behaviour:

    1. entering 'invisible marmalade teapot' and getting no results
    2. changing to 'invisible marmalade teapot with tartan cosy', again nothing
    3. so 'invisible marmalade teapot with tartan cosy in outer space', ditto

you get the idea...? it's just like in the real world, when you might go to a bookshop and say

    You: do you have that new crime novel in stock?
    Bookseller: er, i don't know which one you mean?
    Y: the new crime novel by john grisham, pelican something or other?
    B: oh, right, yes, here it is!

and everyone is happy.

default OR in a search engine would mean my first example eventually starting to return results about transparent space coffee pots with tartan cosies, ignoring the first few terms but the rest match, which is often helpful, particularly if you're doing an exploratory search for something where you aren't sure of the exact details.

`eventually starting to return results about transparent space coffee pots with tartan cosies ... which is often helpful`

I don't find your examples compelling.

If you want to do an additional search that does not depend on your first terms, you simply bring up a new search window.

In your 'real world' example, the equivalent search queries would go something like

search: new crime novel

result: way way too much stuff

search: new crime novel john grisham pelican

result: exactly the right book because every one of those terms applies

Hm. I think perhaps a better way of putting it is that the hard AND issue is when people search using a natural language type query (I know about stop words, assume these are always filtered out) and include some extraneous term, so 'What is that new crime novel by John Grisham about a Penguin I think?' will return nothing, and no amount of extra terms added at the end will help, until you delete 'Penguin'... Of course it's anecdotal, but I still suspect it's one of the reasons for the hard AND to OR switch...