I raised CRS with co2 and did not experience a high mortality rate of shrimplets. It's all about how you balance it. If you inject enough for your plants to properly absorb most, if not all, the co2 injected, then you should be fine. If you overdo it, then I wouldn't be surprised if you experience bad side-effects from co2 injection, such as higher mortality rate. Of course, if you eliminate co2 all in all, you would eliminate the risk of overdosing co2 (even if it's just a little bit OD), so you would obviously see a decline in mortality rate. But if you were to inject a low volume, enough for plants to absorb it, you DO end up with a win-win situation where both plants and shrimp are happy, without any side-effects.
The reason why some people don't bother with co2 is probably because they don't want to deal with the chance of ODing. Fine tuning is required for success, however, co2 injection doesn't always mean lower shrimplet count. It just means you added an extra variable into the shrimp-raising equation that you need to account for, thus, room for error does increase. Simple as that.
Here's a video I made a year ago with my CRS tank with injected co2. Full of babies in it! That tank has been torn down earlier this year, because I moved. However, I hope this is evidence that you CAN have great success, despite using co2.
Ultimately, it is up to the keeper to decide if they want to go through the trouble of finding that "balance" with co2 in their tank. If you are able to tune it properly, you will be rewarded with both beautiful plant growth and lots of babies. If you screw up, you get great plant growth but end up disrupting shrimp population growth. Or you can just ditch the co2 and focus on shrimp and get great shrimp growth and plants are just meh.