Afaic, Smith has nothing left to prove especially not outside Australia. He's made runs vs Anderson and Broad in England, made runs vs Jadeja and Ashwin in India, runs vs Philander, Morkel and Steyn in RSA , runs vs Boult and Southee etc in NZ. Against All teams, he averages 45+ (he averages 30 in 2 only tests vs Bangladesh). In all countries, he averages 40+. He averages 54 away which is more than the other contemporary greats.
That's Sachin level of consistency but more so, he's had more monster series than Sachin vs challenging attacks . I use Sachin because he's imo, the 2nd best after Bradman for now. Some might say Smith didn't face as good attacks and to that I'd say there's no attack from the 90s that was that much better than the combo of Steyn, Philander and Morkel (especially in RSA) and there hasn't been a better spin based attack in my lifetime better than Ashwin and Jadeja especially in India.
That over the next few years, Smith might face less challenges is not unique to him nor should that be held against him if he does well. When all the great bowlers of the 90s retired around the beginning of the 2000s and Pontings, Kallises etc plundered we still gave them their due respect.