They are everywhere. Take opening batters for example (2 in each team) How many end up operating in the slips vs say bowlers?
Also it's pretty amusing your cut off for slips when it comes to what's "decent", but when it comes to 5th bowler anyone who rolls their arm over counts as a measure of skill.
Yes they're there, but are they good?
Ok, you're 5th has never cost anyone a match, they're purely there to supplement your main 4 and help with the rotation, bowl some dog overs and rest your main guys. They're not your primary options, literally not any of your first four. You need a 5th bowler, you don't need a great one.
Decent slip fielders costs you opportunities, actual wickets and games.
In terms of entry level of skill or even the peak level of skill you're attaining in these positions there's drastic differences.
When we're discussing batting all rounders, after Sobers and Kallis it drops off a cliff. It's guys like Simpson and Worrell. There's no elite or match winning levels here among the 5th bowlers.
Next would be the lower order batsmen, and it goes without saying, if any of these guys were elite, or even very good they wouldn't be in the lower order. They definely need more skill to separate themselves from the genuine tail guys, but they aren't going to be confused for top order guys either.
The only area where its a requirement to have a higher baseline requirement is the cordon. If you're average you're a liability. It's also the only skill of the three where you can achieve genuine greatness on the disciple. Sobers wasn't a great bowler, Imran wasn't a great batsman. Waugh, Simpson, Hammond, Sobers, Richardson, Taylor, Kallis were ATG's at the position. Taylor in the slips was a comp to Hobbs as a batsman, not Imran.