Lets say I had a ledger, trezor or any hardware wallet with a seed phrase secured. Let us assume I have fifteen random cryptos stored on it for this question.
Now the company that I bought the hardware wallet from gets nuked in a war and is gone forever.
Will I still be able to access all my crypto? If so how?
Yes, since their software won't be deleted from your computer, nor from GitHub, if their headquarters get nuked in a war.
If the software stops getting development / updates, you need to check if there are alternatives. For accessing the Trezor's Bitcoin funds, you can use Electrum, Sparrow, or even Bitcoin Core. For other coins, it gets tricky; it depends on whether alternative software exists or not. You have to check with the developers / documentation of those coins to get a full answer to this question.