> Just to give you a number: based on the statistics of the Bitcoin Wallet
> app there are at least 2 million wallets depending on BIP37. Not all
> would need instant notification but based on the daily support enquiries
> instant notificaton is the most asked property of Bitcoin.

Yes. Users probably like this feature and client side filtering is not a drop-in replacement for BIP37.

We should also consider:
BIP37 works, because node-operators are willing to offer that service for free (which maybe change over time).
BIP37 consumes plenty of horsepower (disk/cpu) from nodes. Filtering a couple of days of blocks (assume 1000+) eats lots of resources for something, that has no direct long-term value for Bitcoin (the filters data is unique and will be "thrown away“ [can’t be used by other peers]). Same applies for mempool (filtering mempool of a couple of hundred of mb each time the HD gap limit has been exceeded or the app gets sent to the foreground again).

Purely relying on the availability of BIP37 seems fragile to me and start to explore other ways is very reasonable.

/jonas