HTTP pipelining is busted for a variety of reasons. Support exists in most browsers but it's disabled by default because it makes things worse, on balance.
At the time SPDY came out, Opera and Android Browser had pipelining on by default and Firefox was about to also default it on. They didn't only because of the promise of SPDY, not because pipelining "is busted". Pipelining works fine in almost all cases.
And if you only enable pipelining to known-good servers over a non-MITM SSL connection -- exactly like SPDY does -- then there is absolutely no problem with it and it performs similarly to SPDY. But I have no doubt you will continue spreading the party line from your employer, who couldn't be bothered to even test this.
Firefox wasn't about to default it to being on (yes, there was work being done on it to see how workable it was, but there was no decision to ship it) — it's well-known that pipelining causes all kinds of bizarre breakage with badly behaved servers and proxies (and the latter are where the implementations are especially bad).
Opera had enough problems with pretty crazy-complex heuristics as to when to enable pipelining; it would've been nice for them to have been published, but that has never happened. Determining what a known-good server is over SSL isn't that easy.
> Determining what a known-good server is over SSL isn't that easy.
Just the opposite. Both Firefox and Chrome's discussion of pipelining claim that "unknown" MITM software is why they didn't turn on pipelining. Nobody knows what this software is (could be malware). But whatever this mystery software is can't look inside SSL, so pipelining in SSL was just as doable as inventing SPDY.
If Google hadn't pushed SPDY then pipelining was going to happen, and the unknown bad software would have been fixed or blacklisted. Android was using pipelining for years in Browser until Google replaced it with SPDY. Mobile Safari has been using pipelining since 2013 (probably why it wins the mobile page load time benchmarks). Pipelining works.
Yes, some endpoints could be buggy, for instance IIS 4 (in Windows NT 4) was blacklisted in Firefox. Introducing a new, more complicated protocol just because of 10 year old outdated software is not a great way to solve problems.
The endpoints can be (and often are, in absolute terms) buggy. TLS stops bad proxies from breaking stuff, but it doesn't stop endpoints from breaking stuff.
