We have to remember that the 'Prime' missiles are always intended to be test-beds for new techs. When the A-2P was tested back in 2011, many of the techs it demonstrated ended up resulting in the A4 & A5 in the days & year that followed.
He has a view similar to you of the A-2P :
My thought: Agni 2P is a tech-feed prog. With a length of ~20 mtrs, its road mobility is limited. Instead a K4 based missile (with a new nose cone) will be the mainstay for the 2000-3500 km delivery. It's 13 mtrs long, already canisterized and a common base for ASAT, K4 missiles.
— Indranil Roy (@Indrani1_Roy) June 23, 2021
New nose cone ?? New warhead section ? Also another interesting take :
I am now super sure that K4 with a conical or biconical nose is either operational or will get operational pretty soon.
— Indranil Roy (@Indrani1_Roy) June 28, 2021
If the first stage of Agni-P has composite casing (looks like it), then this is the largest built by India.