というものだ

1. 1States what something essentially or truly is, often as a general truth or common-sense definition ("that is what X is").Ex. 1
2. 2Expresses the speaker's evaluative or critical judgment of a situation ("that is (just) X," "that would be X").Ex. 2

It is used in both speech and writing to state an opinion as if it were self-evident. It is common in remarks about human nature, ideals such as love or friendship, and criticisms of behavior that seems unreasonable or wasteful. Compared with plain だ, it adds a tone of conviction, presenting the judgment as what anyone would recognize.

• というものN1「というもの」 attaches only to duration words like 「10年」 and 「ここ数年」, never appears at the end of a sentence, and emphasizes that a state has continued for 'a whole ... this long'; it is somewhat literary in tone. It has nothing to do with the definitive evaluation made by 「というものだ」, so the two cannot be swapped.
• というものではないN2「というものではない」 does not make a definitive evaluation; instead it partially negates a general view ('not necessarily...') and can take the form 「〜ばいい／たらいい」. It cannot be used to deny specific individual facts, so the two are not interchangeable.
• ものだN2「ものだ」 also has recollection (attaching to the た form) and admonition uses, which cannot be replaced with 「というものだ」. When describing inherent nature, 「ものだ」 makes general statements about common sense or expresses deep feeling, while 「というものだ」 sums up the preceding content as its essence or passes an evaluation, often with a critical tone.
• でなくてなんだろうN1「でなくてなんだろう」 reinforces an assertion with a rhetorical question, attaches only to nouns, and is formal and emotionally charged. It is close in meaning to sense 1 of 「というものだ」, which attaches to nouns, but more exaggerated, and it does not suit criticism or dissatisfaction over everyday trivial matters.
• わけだN3「わけだ」 draws a conclusion from already-known facts ('no wonder', 'in other words'), focusing on explaining cause and effect, whereas 「というものだ」 is a value judgment that often carries criticism or emotion, so the two are not interchangeable. When attaching to a noun, 「わけだ」 requires 「な／の」 or the like.
• のももっともだN2「のももっともだ」 first presents the reason and then acknowledges that another person's feelings or reaction are perfectly understandable — 'no wonder...' — in a calm, considerate tone, whereas 「というものだ」 asserts an essence or passes an evaluation, often critically. The former requires 「の」, and nouns and な-adjectives take 「な」; the two are not interchangeable.

Unlike というもの (after a time span, "for the whole past…") and というものではない ("it's not necessarily so"), というものだ asserts what something truly is. It adds a labeling judgment beyond ものだ's general truths, is less rhetorical and literary than でなくてなんだろう, and unlike わけだ (logical conclusion) or のももっともだ ("it's understandable that"), it states the speaker's own evaluation.