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I'd guess that RCA and Armstrong were too busy suing each other to do that.
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Actually the acrimony was just beginning. The law suits came later.
I am only surmising but it does seem credible that RCA traded acceptance of FM for TV in trade for frequency band allocations (42MHz to 50MHz) for FM Radio broadcast in 1941. Remember, early 40's technology dictated lower frequencies and RCA and the NTSC gave away a chunk of bandwidth which could have been used for an extra TV channel.
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FM can live within the NTSC video sideband, but AM cannot as it would pick up the video highs. The AM sound carrier would have to be located outside of the video sideband and that would require a wider band width.This is like phase modulation used in NTSC color can live within the video sideband and (with a little luck) not be messed up by the AM video.
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I do not believe this to be so. The BBC experimented with NTSC color in the fifties and sixties for the 405 line system with no change in channel or audio parameters. The audio carrier is located entirely separate from the video in both 525 line with FM sound and 405 with AM sound. Adding color is a matter of using suppressed carrier quadrature modulation and frequency interleaving in the video band only.