I'm not talking about the number of fully-established sides or comparing Pak's ranking in the 1950s to BD's ranking in the 2000s. What I'm saying is Pak still had to play fully-established sides, even if there were only 2 or 3 of them and did well. Even if you exclude BD's performance against the top 5 test teams in the 2000s and look at their performance against only the No.6-8 teams, their performance was pretty mediocre. You can't say that Pak had it easier against Eng in the 1950s than BD against SL in 2000. SL had made their Test debut only 18 years prior.
I don't think BD had it any easier against SL than Pakistan or India had against England. Playing
in England/Australia is a different thing. Asian teams still struggle there.
But as oppositons, I think SL was very strong by 2000. Because of all the teams that have played cricket so far, Sri Lanka's growth as a cricketing nation was by far the fastest. They had already won a WC, a CT and 2 Asia Cups by year 2000.
Their 2000 team already had WC players & ATGs like Ranatunga, de Silva Jayasuriya, Murali and Vaas. And at the turn of the century they already had younsters like Sanga, Jayawardene and Dilshan coming in.
SL was already very strong team when BD came in to the picture. And they maintained that strength all the up to 2015.
BD, in its first WC appearance in '99 caused an upset over Pakistan. And '99 Pak was a lethal ODI team. In 2005 they beat Australia. In 2007, they beat India in the WC. All 3 of those wins coming overseas, nonetheless. Even back then they were pulling rare victories over much stronger teams.
If you take India and Pakistan's record against AUS, WI & ENG in their first 2 decades of test cricket, it's a W/L ratio of 0.205 for India and 0.263 for Pakistan.
By the time BD made their test debut, teams like England, Australia, India and Pakistan had progressed too much (in terms of popularity of the sport, monetary investment, infrastructure, etc).
But against teams that BD realistically could compete with (namely SL, WI & ZIM) they have also maintained a W/L ratio of 0.272. That's quite respectable.
You must understand that the level of international cricket changed dramatically from 1950 to 2000. By 2000, cricket had become a lot more professional. There was a lot more science and medicine behind it. Tons more investment and man-hours (support staff, psychologists, media manager, this that). The rate of growth of global cricket in the last 30 years is infinitely more than it was between 1900 to 1960.
The environment that BD made their test debut in was far more difficult because they had to catch up a lot more.
This is true of any field in life. Even in the business world, if you begin early you will always have first movers advantage. And as time goes on, it will become more and difficult for the new market entries to catch up with the first movers.