You're describing a rule of English grammar called Comparative deletion.
This is not an optional feature. When using most comparatives, you must delete the object noun (or noun phrase) from the clause that follows "than". The reason behind this is that, in a comparative where both clauses will have the same object, stating it in both will requires either a repetition, or a redundant use of "it".
Where the verbs are the same, it's commonplace to either replace the verb in the second clause with "to do", or to delete it entirely. The examples 1, 2 and 3 below all have the same meaning: 1 would be rare, 2 is common in speech and informal writing, 3 is common in formal speech and in writing.
- Correct He always spent more money than George spent.
- Correct He always spent more money than George did.
- Correct He always spent more money than George.
- Incorrect He always spent more money than George spent it.
You can have comparisons where the two clauses have different verbs, as in:
- Correct He always spent more money than George saved.
However, there are comparatives which have different objects, and in this case, obviously, you should not try to drop the object from the second one. Consider:
- Correct He scored more goals than George made saves.
In this case, only the count of the things is compared (the number of goals he scored, versus the number of saves that George made).
This type of sentence (number 6) is probably where you got the idea that both halves of a comparative needed to be complete clauses, but that is not a rule in English grammar. In fact, where the clauses share the same object or the same verb, the rules of English grammar require you to make the second one "incomplete".
However, be careful to not do this if it will make the meaning of the sentence ambiguous, especially when you delete verbs, as in this example:
Original: The dog loved his owner more than the cat did.
Ambiguous after deletion of verb: The dog loved his owner more than the cat. (Can now also be read as saying that the dog loved its owner more than the dog loved the cat)
Regarding your second sentence, it's of the "number 6" type, simply comparing the magnitude from the first clause with the magnitude of the second clause. The first clause is "wealth", and the second is "things there are to spend it (wealth) on". In this, the "it" cannot be omitted without confusing the reader.