If only we could have had this back then, it would have been good
"If only we could have had this back then"=「もし当時、私達がそれを持つことができていたならば」
"it would have been good"＝「それはよかったのに」
または、"things would have been better"＝「状態はもっと良かったのに」
または、"we could have used this (and then things would have been better)"＝「私たちはこれを使うことができた」
It would have been good if they had a product like this back in the day?
この場合、次のような言い方ができますよ。
ーIt would have been good if they had a product like this back in the day?
「当時、こんな商品があったらよかったのに」
it would have been good if ... で「…なら良かったのに」
back in the day で「当時・昔」
ご参考まで！
アメリカ合衆国 
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