Why does India buy Russian weapons & can it operate well without them?

India and Russia have had a long-standing defence relationship since 1960s,  India has purchased the majority of its arms from Moscow, and there are various reasons for this. Prime Minister Jawaharlal Nehru’s anticolonial sentiments set the country on a path to autonomy and self-reliance, but Defence Minister Krishna Menon's socialist leanings moved the country closer to the Soviet Union.India's nonalignment strategy aimed to counterbalance its legacy defence and bureaucracy links with the British and the West by actively building defence ties with the Soviet Union. However, the US–Pakistan alliance was far more crucial. While Nehru was wary of the US and did not want to be drawn into the Cold War by supporting with either the US or the Soviet Union, the US's support for Pakistan pushed India's hand.The United States' collaboration with Pakistan, first through Southeast asia treaty organisation and subsequently the Baghdad Pact in the 1950s, naturally drove India to lean toward the Soviet Union to counterbalance Pakistan.
Congressional Research Service (CRS) report has said Indian military cannot operate effectively without Russian-supplied equipment and will continue to rely on its weapons systems in the near future and added that New Delhi is going “full steam ahead” with the induction of S-400 and the the first deliveries are set for autumn 2021, to be completed by early 2023.
why Indian military cannot operate effectively without Russian supplied equipment
, why is it that the Indian military is unable to function efficiently without Russian-supplied equipment?
While India has expanded its supply base to include Israel, the United States, and France, Russia remains a major supplier.
Russia supplies 86 percent of India's military equipment, weaponry, and platforms.
The INS Vikramaditya, the Navy's only operating aircraft carrier, and Chakra II, the Navy's only nuclear attack submarine, are both Russian.
So are the Army’s The main battle tanks T-90 and T-72 as well as the Su30 MKI fighter of the IAF
BrahMos, the country's only nuclear-capable supersonic cruise missile, is developed in partnership with Russia.
India's reliance on Russia for military weapons is due to a variety of factors.
legacy issue: : India and Russia have a long-standing defence partnership, and their methods and systems are familiar to one other.
Specialized Equipment: Russia gives India with specialised equipment that sets it apart from other countries, such as the S-400 Air Defense Missile Systems, nuclear submarines, and aircraft carriers.