really it's speculation, but isn't everything when talking about the future? you can never say for sure what will happen just make educated guess's. in ODIs especially BD is going to be a much more experienced team in 3+ years than a lot of other teams.
in tests the experience gap won't be as much but that's mainly because BD plays half the amount of tests (or less) per year than the other test teams do so it's going to be easy for the teams to catch up on that experience in tests.
but it's quite possible just based on current stats that BD will be at least equal in batting to teams like SL (when the big names retire) and NZ. SL will be relying on mathews, chandimal and thirimanne. mathews really should have established himself as a quality test batsman if he was going to by now, not saying he won't but obviously his progress hasn't been as fast as expected so he's not that far ahead of guys like tamim or shakib and actually atm i might even select those 2 ahead of mathews as it is right now. chandimal might become something but he hasn't played enough tests for us to see whether his rise will be quick, steady or slow and thirimanne has the talent but even in ODIs has struggled to put the scores up so the jury is still out on him.
NZ only have ross taylor, i don't see NZ producing a better batsman than him atm, but they've only got one of him and the next best are averaging low 30s or lower, even the experienced ones.
meanwhile BD has tamim, shakib, nasir doing quite well in tests, riyad has comeback to some form, mushy is no worse than any NZ batsman bar taylor.
i know i'm just talking about batting but when it comes to talking a test match to the 5th day it's more about batting than bowling, the more capable your batting is the more runs you'll score, the more time you'll spend out there making a good score and the less time you give opposition to bowl you out. anyway it's not like SL or NZ are really producing amazing quicks or anything.