You basically *need* combat arms to get past O4.
And whoever told you a "lot" of senior NCO's aren't "up to snuff" is probably an officer that didn't like being put in his place by an NCO that made him feel stupid.
You don't get to be a senior NCO by being a screw up because somewhere along the line you will be discovered and simply will not be promoted. Think about it. You don't get to E5 due to time in service. You have to actually take and pass at least PLDC. And you only get recommended for PLDC by being a solid and promotable E4 since each unit has a finite number of slots allocated [my unit had ONE and gave it to me - because I had them fooled!]. Now think further down the line. How many E5's are there vying for a single E6 slot? Even if you somehow fool the system for, technically, a second time [getting promoted to E5 in the first place] what are the chances that as a screw up you are going to beat out a high speed candidate for those super rare E7 slots? I will buy that there is a small segment of E5's that just don't pan out as expected. Even if that number is as high as 25% [and I highly doubt that to be the case based on my experience], those low performers aren't going to be recommended for BNCOC [ALC] or ANCOC [SLC].
Conversely, O1-O3 is pretty much promotable due to time in service. My sister was promoted to O3 while serving the IRR portion of her contract! O1-O3 are typically XO's, and Company Commanders. Thus, you have a lot of screw ups running military organizations in a very public way. It isn't until vying for that O4 position that you really need to be stellar. And at O4, you're probably getting out of the field and high enough up that you aren't really interacting with enlisted troops all that much anymore.