I'd say both the Enclave and the Tahoe LT w/ Nav are legit upgrades with no LCARs available. I wouldn't say that either is any worse than a Cadillac SRX or Lincoln MKT, both of which I've received on a LCAR res when none were available. A PGAR/FFAR/PFAR like the Tahoe is also an upgrade over LCAR in the technical sense and almost always higher in rate also (though, as a counterpoint, an MVAR is as well). I agree it's not what the OP reserved and he should complain both to local management and corporate customer care if he wasn't happy with the alternatives that were available.
The issue with the BMW 7 or Jaguar is they likely fall into the "Exotics" category and a free upgrade to such is likely impossible for a front line employee to do and a manager overriding it would probably draw attention from higher ups. Your anecdote of receiving a 5-series (likely a 528i with leatherette and no navigation) doesn't really compare to receiving a 7-series which is a vastly different car in price, features, and category.
I hate when my reserved car class is not available too, and I would probably TRY to get the crazy upgrade as well, but I wouldn't be shocked or infuriated when it didn't happen... the mentality of "my reserved class isn't available, I should get whatever on the lot catches my eye" just isn't workable and shouldn't be the expectation.