"Are there any benefits to having package not be a lexical boundary?"

It's sometimes handy to switch package mid-sub...

use v5.12;
use Data::Dumper (); # not importing Dumper function here

sub d
{
    package Data::Dumper; # please don't clobber @_
    local *Indent = local *Terse = 1;
    say Dumper(@_);
}

d [1, 2, 3];
d {foo => 123};
[download]

Also if package touched lexical variables it would probably confuse newbies into thinking that namespaces and lexical scopes have something to do with each other.

/div