も当然だ details

Examples

it's only natural that; no wonder — given the stated circumstances, a reaction or result needs no further justification

あんなにひどいこと言いわれたら、怒おこるのも当然とうぜんだよ。

anna ni hidoi koto iwaretara, okoru no mo touzen da yo.

After being told something that awful, of course you'd get angry. casual

準備じゅんびもせずに試験しけんを受うけたのだから、落おちたのも当然とうぜんだ。

junbi mo sezu ni shiken o uketa no dakara, ochita no mo touzen da.

Since he took the exam without even preparing, it's only natural that he failed. neutral

長年ながねんこのプロジェクトに尽力じんりょくされてきたのですから、皆様みなさまが惜おしまれるのも当然とうぜんでございます。

naganen kono purojekuto ni jinryoku sarete kita no desu kara, minasama ga oshimareru no mo touzen de gozaimasu.

Given how many years you devoted to this project, it is only natural that everyone should be sorry to see you go. formal

The の nominalizer before も is obligatory — ×怒おこるも当然とうぜんだ
drops it and is ungrammatical; only 怒るのも当然だ works.

だけのことはある
だけのことはある judges a specific person or thing as living up to a known reputation or effort, and stays admiring (三年さんねん勉強べんきょうしただけのことはある); のも当然とうぜんだ judges a REACTION as unsurprising given the circumstances, with no admiration required — 怒おこるのも当然とうぜんだ excuses anger, something だけのことはある could never do.
はずだ
はずだ predicts from evidence or calculation what SHOULD be true, and can be proven wrong when reality defects (来くるはずだったのに); のも当然とうぜんだ never predicts — it retroactively judges a reaction or result everyone already knows about as unsurprising, and is never falsified by events.