Ground launched SDB with old M26 rockets is a little redundant with GMLRS-ER. Plus those motors have to approaching their sell by date.
Not really. MLRS/SDB is far more versatile than GMLRS-ER. More range, better warhead, more approach angles, etc. etc.
It's not that clearcut.
Range for MLRS SDB is quoted as 150km, which is the same as GMLRS-ER. Warhead in an SDB 1 is 206 pounds with either 36 lbs of HE (penetrator warhead) or 147 lbs HE (low-frag blast warhead) while GMLRS Unitary and the ER version are using a ~195lb warhead with 51 lbs of HE. And they have demonstrated vertical arrival with GMLRS-ER.
So basically, GLSDB gets you some ability to do very low angle arrival for stuff like cave entrances and maybe to fly dogleg approaches. But it's also slower than a semi-ballistic GMLRS-ER round and more mechanically complicated because you need to separate the booster and unfold the fins in flight, etc.
I'd love to see unit costs for both but it is really hard to find apples-to-apples comparisons.