I`ve seen this graphic posted on some other forum thread before.
There`s plenty of space for bitcoin to grow,but it`s wrong to compare it to gold.
Gold is used by the people for centuries.All the people without computer knowledge will trust gold,and believe me,most of the people in the world don`t have a PC,which means that they don`t know anyhting about btc and most likely they will never buy and use bitcoin.
Median global computer ownership was at 38% in 2015, it's not hard to imagine it going to 70-80% in next 20-30 years. Which will mean that Bitcoin can easily become a gold of digital era, even if it won't scale into a currency level. And Bitcoin is more convenient for private storage and moving, it can be used anonymously, it can be hidden from all other people. Gold is valuable because it's scarce and chemically durable, nothing more, so Bitcoin can also be extremely valuable, because it's scarce and digitally durable.