Around that time SPB had some throat problems(blisters due to smoking and incessant singing in 3+languages) too and had to cut down on his singing - a reason why "track" singers became main singers in that period. I dont think Raaja purposefully sidelined anybody from what they do their best. Let us also bear in mind that voices like SPB, KJY and MV are special, close to late 80s and 90s, exectations were always there to use their voices mainly for reigning stars (exception being Kamal who had hit the jackpot with his "track song" Raja kaiyya vecha - Anyone who had echo cassettes of Apoorva SagodharargaL would have had the pleasure of hearing SPB sing Raaja Kaiyya vechchaa).