We were kept waiting longer than was absolutely necessary.

...than it was absolutely necessary that we should be kept waiting.

The town offers better cultural advantages than has been generally supposed.

...than it has been generally supposed that it can offer.

That morning we drank more tea than had been our custom.

...than it had been our custom that we would drink.

She found the house much more luxuriously furnished than suited her purpose.

这个句子本身就是一个完整的比较从句，无须也无法再补全。than作为比较代词，本身就隐含了它所替代的the house which在内了。如果将than分析为连词不作成分，则主句比较短语的对应项在从句中作主语，该主语是必须省略的，不可能出现。故不能补全。