Just too many drawbacks.
For server to server, they might be fine, but to client just stick with JSON. (which when compressed is pretty efficient).
I did this one for reading json on the fast path, the sending system laid out the arrays in a periodic pattern in memory that enabled parseless retrieval of individual values.
Also I assume you'd have to have some sort of binary portion bundled with it to hold the field offsets, no?
I think I'd take a different approach and send along an "offset map" index blob which maps (statically known in advance based on a schema that both client and server would need to agree to) field IDs to memory offsets&lengths into a standard JSON file.
Then you have a readable JSON, but also a fast way O(1) to access the fields in a zero-copy environment.
Done right the blob could even fit in an HTTP response header, so standard clients could use the msg as is while 'smart' clients could use the index map for optimized access.
But as I said would suffer from numeric values being text encoded. And actually 'compiling' the blob would be an extra step. Wouldn't have all the benefits of flatbuffers or capnproto, but could be an interesting compromise.
I'd be surprised if this isn't already being done somewhere.
Also browser has JSON parsing built in. Less dependencies. Easier tooling overall.
In my experience people overuse protobuf. But I also worked at Google, where it's the hammer in constant search of any nail it can find.
At the very least, endpoints should provide the option to provide a JSON form through content representation negotiation.
We moved to flatbuffers and back to JSON because in the end of the day, for our data, data compression with JSON+gzip was similarly-sized than the original one (which had some other fields that we were not using) and 10-20 times faster to decode.
That said, the use case for flatbuffers and capnproto isn't really about data size, it's about avoiding unnecessary copies in the processing pipeline. "Zero copy" really does pay dividends where performance is a concern if you write your code the right way.
Most people working on typical "web stack" type applications won't hit these concerns. But there are classes of applications where what flatbuffers (and other zerocopy payload formats) offer is important.
The difference in computation time between operating on something sitting in L1 cache vs not-in-cache is orders of magnitude. And memory bandwidth is a bottleneck in some applications and on some machines (particularly embedded.)