I published a few studies across all positions. DL (iDL and Edge), and RB (and QB) had higher bust rates (10 and 25 year samples).
For the info above to have any legitimate meaning, it would need to include all positions and compare accordingly.
The last two years that you list have an 80% hit rate. That's probably pretty good vs other positions. It also counters your point about how much they may not be ready for the pro game.
In terms of average taken WRs in the first, just based on pure numbers (snap counts, positions needed including STs, standard depth by position/rotation, ), we should expect on average of 3.5ish WRs taken per year IIRC.
I'd also add that years fluctuate (numbers of positions taken) based on talent available, league O or D trends, etc.. This year will likely skew Edge and iDL a bit, just like 2014 did for WR. It happens, and will continue to happen. And we also know many positions have been devalued and overvalued through the past 10 years.
In terms of being imperfect when they enter the league, most rookies are. All players can be limited by their systems in college.
As far as how you classify success, you don't define. You don't speak of injury impact either. Fuller for instance, who you call a miss, still has averaged 500+ yards a year with serious injury issues across multiple seasons. That's still a higher average than every Colts WR not named TY.
In short, if you're not comparing to other positions, or defining success criteria, not much to take from the above.