Yeah that's exactly the problem really. I can't see one slip fielder being as valuable as a good lower order bat. There's just not going to be enough chances created per match that go to one particular fielder for it to match a lower order batsman's production.
That's until a catch is dropped and the course of a game changed.
The 5th bowler doesn't bowl every game far less take a wicket, no. 8's have games where they don't reach double figures in the first and don't get to bat the second innings. Imran played 88 tests and batted in 126 innings, and the further down the lineup, the worst that ration becomes.
It's less individual games and more impact over the length of a series.
All 3 aspects have their caveats.
Lower order batting is considerably less reliable than the top order and more likely to to shine in draws than feature in wins, additionally they don't pad up nearly as often.
5th bowlers is an automatic down grade to the primary operators and most don't approach e en 2 wickets per match. They're the weak link to an attack that generally gets the worst conditions of the ball and often match scenarios.
With regards to your point, the most important slip and the one comparable to the no. 8 / 5th bowler, is your second slip, who moves to first for the spinner. and yes, they generally do get enough chances to compare make them comparable in importance to either. They get the most chances and the more difficult ones. And even at third where they get the least amount of chances, they're the most difficult and are just as critical to take.
You've advocated batting down to no. 11, they get even less batting opportunities and if your no. 11 is consistently batting in critical situations you're likely loosing anyways and you have bigger issues.
TL. DR, yes tour 2nd slip is utilized enough to match the value of your no 8 batsman.
And unlike less consistent lower order batting which declines with upscaling of competition or difficult conditions, elite slip fielders don't suffer from that, and are more consistent and reliable.