Some terminology: Hiragana and katakana together (as opposed to kanji) are called "kana". They may have used "hiragana and katakana" so as not to bombard you with new terminology. Also, "gairaigo" is a Japanese term for loanwords from western countries.
I don't know an awful lot about English. I'm merely a native speaker of it. So I can't really comment about loanwords in English, except to quote the following from James Nicoll:
The problem with defending the purity of the English language is that
English is about as pure as a cribhouse whore. We don't just borrow
words; on occasion, English has pursued other languages down alleyways
to beat them unconscious and rifle their pockets for new vocabulary.
But there are some words in Japanese that don't have a non-gairaigo form because they were invented in European countries. As an example Is タオル used for the towels used at onsen? had the following as an answer:
Towels were introduced in the Meiji era and were almost exclusively
imported from England. That's why the word タオル came from English, and
there is no other word for them (although Japan of course had their
own fabrics before that (I'm specifically talking about terrycloth)).
Regarding "getting by". While in beginner textbooks there's no kanji, a lot of hiragana and a lesser amount of katakana, in my experience of Japan as a tourist, there's a lot of kanji, a fair amount of katakana, and a small amount of words made up entirely of hiragana. There's some words made up of kanji plus hiragana, but knowing only the hiragana part won't help you much.
The main time I've noticed hiragana in Japan are words like です and ください and さん. I didn't notice many nouns, or many verbs being written entirely in hiragana, though I could be mistaken.
A major benefit of learning hiragana is to help with learning how to pronounce Japanese. That's because the relationship between how something is written and how it is pronounced is straightforward for Japanese written in kana. The problem with Romaji is that you have to remember how "a", "e", "i", "o" and "u" are pronounced in the context of Japanese, as opposed to in English.