Would you not be able to fly <FL290 i.e. non RVSM and fly beacon routes? As someone said, just like the old days. There are plenty of non RNAV & BRNAV a/c flitting around the blue yonder. It's only a computer that makes life easier. It is not a 'must have', but a 'nice to have'.
There was case a few years ago, before the extensive use of RNAV & BRNAV, when an EFIS IRS FMC a/c took off from a northern London airfield to fly to a N.W England airfield on a clear day. VNAV, LNAV, A/T would not engage. I suspect the FMC had gone on holiday. The capatin returned to departure airfield with his 'broken' a/c. Chief pilot asked, "why?" Quite right too.