This has been discussed before, but here's a good opportunity to point this out again.

In his book Jane Austen's Novels: The Art of Clarity, Roger Gard has a chapter entitled "Implications of the Second Chapter of Sense and Sensibility".

The scene where Fanny talks John from giving his sisters £1000 each down to nothing is one of only two departures in the novel from the action being seen through Elinor and filtered through her consciousness.

Gard writes that this "piece of bravura comedy" is "hardly surpassed anywhere in literature for its ruthlessness" and that readers remember the specific scene in Ch. 2 long after "they have forgotten all the limpid and delicate pros and cons about prudence and feeling, reason and romanticism." He says that people connect it to the title and theme of the novel as "a delicious preliminary negative definition of what, if overdone, being sensible may become."

But, he says the scene also has a "comical/awful logic" and "lethal rationality" nearly identical to this scene from Act II, scene iv of King Lear:

KING LEAR

I can be patient; I can stay with Regan,

I and my hundred knights.
REGAN

Not altogether so:

I look'd not for you yet, nor [now] am provided

For your fit welcome. ...
KING LEAR

Is this well spoken?
REGAN

I dare avouch it, sir: what, fifty followers?

Is it not well? What should you need of more?...

Yea, or so many, sith that both charge and danger

Speak 'gainst so great a number? How, in one house,

Should many people, under two commands,

Hold amity? 'Tis hard; almost impossible.
GONERIL

Why might not you, my lord, receive attendance

From those that she calls servants or from mine?
REGAN

Why not, my lord? If then they chanced to slack you,

We could control them. If you will come to me,--

For now I spy a danger,--I entreat you

To bring but five and twenty: to no more

Will I give place or notice.
KING LEAR

I gave you all--
REGAN

And in good time you gave it.
KING LEAR

Made you my guardians, my depositaries;

But kept a reservation to be follow'd

With such a number. What, must I come to you

With five and twenty, Regan? said you so?
REGAN

And speak't again, my lord; no more with me.
KING LEAR

Those wicked creatures yet do look well-favour'd,

When others are more wicked: not being the worst

Stands in some rank of praise.
To GONERIL
I'll go with thee:

Thy fifty yet doth double five and twenty,

And thou art twice her love.
GONERIL

Hear me, my lord;

What need you five and twenty, ten, or five,

To follow in a house where twice so many

Have a command to tend you?
REGAN

What need one? 

Gard goes on to say:

The real charm of this slightly insane logic (of John and Fanny's) is that there is nothing really wicked about it. It resembles that of Lear's daughters in having its own internal consistency and reductive drive.

John's "lukewarm goodwill" is realistic and a kind of "respectable baseness".