I already explained this to healer-san in his previous thread, but the stem of a typical na-adjective doesn't perform as a noun. Of course there are na-adjectives whose stems are a noun, but it's not applied to all na-adjectives. Please refer to the following posts.
So my girlfriend (not me, I promise) likes J-pop - I heard her listening to Koda Kumi's 'Ecstasy' the other day - and the grammar of the first phrase confused me (like most music). The first phrase is 'かけがえのない今をもっと深く' which some websites translate as 'The present can’t be replaced'. Kotoba...
jref.com
Hello! I know the adverb 一々 means 'one by one/separately'. Could someone please give me an example of a sentence with ichiichi in it? Thanks a bunch ! :D
jref.com
大変 belongs to typical na-adjectives as same as 静か, so the suffix さ is needed to change the stem into a noun.
×大変がある。
○大変さがある。
×大変を減らす。
○大変さを減らす。
Incidentally, even when the stem of a na-adjective can work as a noun, the suffix さ can add a different meaning to the stem as a noun. For example, while 自由 means "freedom/the state of being freedom", 自由さ can mean "the degree of freedom", similar to 自由度.
e.g.
戦前と戦後では自由さが違う。
戦前と戦後では自由が違う。is also grammatical, but the meaning is not the same as the original ("the degree/level of freedom is different" vs. "freedom is different").
Similarly, 健康さ means "the degree of health/how healthy the mackerel is" in the following sentence, and it can't be replaced with 健康.
目利きマイスターは、市場に陸揚げされた八戸鯖の姿を見るだけで、脂の乗り具合や新鮮さ、健康さを見分けることができます。