Option A seems quite possible, but Option B is more likely!
Conventional Locos are not as energy efficient as 3-phasers. Besides that if CLW aimed for the P7 to replace the P4, then they will do it........and it will happen sooner or later.
Already P4 production is being decreased while P7 production is on the rise!
And taking a taking into account that the...
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WAP-7 & WAG-9 come out from the same assembly lines, it won't take long for CLW to amp up it's production in numbers.
Besides your assumption that P7 is heavier than P4 is wrong. P7 was built with special light-weight alloys to be lighter than the P4, I don't know their exact weights but the P7 is lighter than the P4, that I am very sure about.
Besides that IR's future plan seems to include the P7 and P5 as the major locos.....P5 for high speed & P7 as the workhorse locomotive.CLW had a well-set plan of completely switching over to 3-phasers soon.However some stubborn railway zones like SER (I am not an SER hater), which find themselves unwilling to accomodate, and adapt to 3phaser passenger locos, complicate CLWs problem!
If every thing goes accordingly to plan CLW is likely to stop P4 production completely by 2025........
However you are definitely right in pointing out that the P4 won't completely go out of service. The P4 is a too fantastic loco to be ditched completely, and considering IR's conservative mindset, and it's habit of treasuring it's old locomotives (Read WDM2 which were supposed to be completely phased out in the 1990's but can still be found almost everywhere. I am not a WDM-2 hater as well (Actually WDM-2 happens to be one of my fav locos & it is the IR loco king for me......but it is outdated from today's point of view.).)
PS:- I firmly believe the WAM-4 will actually outlive the WAP-4........It is just one of those constant things in a ever-changing IR world.
With it's 24 coach hauling ability, working capacity on steep gradient-change areas, & 110-120 MPS the WAM-4 is still as relevant today as it was, when introduced in 1970! It may consume more power, but its extremely high reliability more than makes up for it!