If I understand it correctly, Lisp-2 is a way (not the only way!) of maintaining more intuitive semantics in the face of macros. If I write the following:
(your-macro
(let ((x (foo y))
(bar x))))
I probably don't want the meaning of FOO or BAR to be up for grabs based on the expansion of YOUR-MACRO, and certainly not LET. Lisp-1s often have hygienic macros to deal with this concern, but not always.
Completing your thought (please let me know if I misunderstood you) Y _is_ up for grabs because what you wrote might macroexpand to for example (let ((y 5)) (let ((x (foo y))) (bar x))).
Where your comments falls down is that all the Lisp-2s I know allow the "function definition" of a symbol to be overriden in a similar way, e.g., in Common Lisp the macro might expand to (flet ((bar (arg) (* 5 arg))) (let ((x (foo y))) (bar x))) where FLET is a macro similar to LET but for function definitions.
Where your comment falls down is that in a Lisp-1, every let is also a flet.
Also, of course, you must use gensyms for any locally bound identifier, whether it is a let or flet. Nobody said that Lisp-2 allows for labels and flets without having to use gensyms; nobody in their right mind is going to lexically bind identifiers in macro-generated code that don't use gensymed names (other than in cases when there is no possible capture).
Lisp-2 addresses (in a good-enough-beats-perfect way) the following problem: the programmer's code wrongly capturing references that the macro would like to use.
The macro wants to generate some (foo ...) call to a function in the global environment. But, oops, in a Lisp-1, a user's local variable foo takes this. In a Lisp-1, the user would have to have a local function by that name.
If global functions have reasonably descriptive names, and local functions are used sparingly, the potential for a clash is low.
We can have a warning when a local function shadows a global one; it won't be too much of a nuisance since global functiions tend to use descriptive names, and local functions are relatively rare compared to local variables.
Yes, that's a good point. Multiple namespaces arose in Lisps before things like FLET and LABELS, so this particular issue is likelier to come up in Common Lisp nowadays. I think there are several historical considerations in play and https://www.dreamsongs.com/Separation.html (as mentioned) is probably the best source for more information.
