IMHO the XJO is about 10% below the very long term average(very long as in the last 35 years). So if you can take a long term view, i.e. > 5 years, XJO should outperform its own historical annual gain of about 9-12% for the next 5+ years. But bear in mind that within that 5+ years, there could of course be years where it goes backwards.
Kind of treat it like residential property where historically, it has gone up by about the same percentage.