Endianness is an issue because programmers ignore it - they think "undefined behavior" is a synonym for "not yet standardized", and the mentality of "works on my machine" typically trumps concerns of portability.
This isn't a concern for low level developers, such as the kernel developers - they understand the concerns and take care to implement code in portable ways.
The issue is with user-space developers who think C and C++ are a good choice of language, and they have no qualms using bitfields, unguarded compiler pragmas, violating strict aliasing rule, and failing to specify the endianness their protocols use in the protocol itself (BoMs are not universally used) - also there is often a failure to provide the endianness conversions in implementations of such protocols where necessary. Not to mention a complete lack of standard way to test the endianness of the current machine, which typically requires violating the strict aliasing rule to check.
I believe it's safe to assume having more endian-diversity is then a good thing. Bugs in software and protocols will be exposed and eventually corrected.
Since most Linux distros fully support a very diverse set of machines, endianness is usually not a problem with most of the software that's already part of a Linux distro.
As for software developed inside Google, they hire smart people. They'll manage.
This isn't a concern for low level developers, such as the kernel developers - they understand the concerns and take care to implement code in portable ways.
The issue is with user-space developers who think C and C++ are a good choice of language, and they have no qualms using bitfields, unguarded compiler pragmas, violating strict aliasing rule, and failing to specify the endianness their protocols use in the protocol itself (BoMs are not universally used) - also there is often a failure to provide the endianness conversions in implementations of such protocols where necessary. Not to mention a complete lack of standard way to test the endianness of the current machine, which typically requires violating the strict aliasing rule to check.