All you need to do is to back up your blob with an AT gun and the Flak HT is completely shut down.
No it isn't. It depends on where you put your flak HT silly.
Also if you flank the AT gun, you can threaten it and the blob won't be able to retaliate because it will be suppressed.
It's like saying that if you have a machine gun behind your blob, infantry are useless. The difference here is that a mistake with vehicle positioning is punished much more harshly than one with infantry, but the concept is exactly the same.
That said, this is why the kubel was awesome. AT guns had a hell of a time hitting it giving you more time to suppress + eliminate the AT gun. The flak HT isn't as lucky, unfortunately.