As per @koxinga’s post it isn’t only American kit but also Euro kit
India is interested not only in outright buys but the technology which comes along with it. I can think of only a couple of European countries willing to share the technology to the extent that India desires. You'll have noticed that India is very selective when it comes to buying European and there's a reason for it.
When it buys Russian there are no worries about embargoes and sanctions and no worries about stuff being downgraded; the same can't be said of buying American and European. France would have been more than happy to sell ASTER 30 and the Americans might have been willing to sell Patriot yet the Indians decided on the S-400. There's a reason for this.
As for maintaining a relationship with a UNSC member generally supportive of Indian interests
It goes beyond that. If you read the link I posted it explains the strategic relationship and why the mutually beneficial relationship has endured
yes that is important but not at the expense of equipping your military with obsolete/vulnerable kit.
What "
obsolete/vulnerable" kit has India been buying in recent years? The IAF is very happy with its Su-30s which it has modified with local kit and operates at a systems level. The IN has no issues with its Russian kit and the army; despite being able to source alternatives still decided on various Russian kit which it has no major complaints about.
What it can't source from Russia it sources from elsewhere; when the IAF needed a replacement for its MiG-23s/27s it selected Rafale - part of the reason is that the Su-30 isn't suited for the low level role which the MiG-23s/27s performed and a large part was because India for strategic reasons has always desired a Eastern/Western mix.
If anything it's inherent issues dealing with the Russians rather than Russian kit per see which has been India's main source of grievance. Apart from South Korea another country which on paper is an ideal partner for certain things is Turkey [sone of the stuff the Turks are doing is impressive] but I have no idea how chummy the Turkish/Indian bilateral relationship is.