I don't know if he will have a long term future here, but I can see him interesting clubs like Milan, Atalanta, Napoli, Roma in a season or two time. Along with most of the EPL teams.
Would not surprise me either if we see Dest play most of the season in the attacking position.
I mean he's not a Dani Alves tier of RB, but as long as we don't stumble upon one, which is rather unlikely looking at best fullbacks in the last few years, Emerson has a very good chance to be here long term. If he can transfer his performances for Betis to Barcelona he's easily our best option. Not that it's a given though - see Firpo. But I have the hope Emerson's mobility sets him apart. Junior is rather stiff in comparison.