First of all it's not on life support. I've taken over maintaining it for not only FreeBSD but sending patches to NetBSD and Illumos have expressed interest that I update their codebase.
People say this because they haven't used it. It's like people tell me FreeBSD is not secure because it's not that BSD or this Linux
Darren stopped working on it when he left the employ of Sun Microsystems. That was when Oracle purchased Sun.
As to performance, IPF and PF perform better than IPFW when NAT is involved. Packets cross kernel userspace barrier and back again through NAT. IPFW NAT is inefficient. In that regard PF and IPFILTER are of similar design.
PF was written by OpenBSD because Theo and Darren had a falling out. Theo ripped IPFILTER out of NetBSD, writing his own, PF, over the space of a week.
IPFILTER's pools (hashes and tables) are more advanced than PF's. Though PF has gained that capability recently. I don't think it's nearly as useful yet.
As to how IPFILTER an PF operate, both are similar -- remember Theo wrote PF to replace IPFILTER after the dispute.
I've been using IPFILTER for close to 30 years. Initially on Solaris and now on FreeBSD. It does the job quite nicely and IMO I prefer using it over Checkpoint (firewall appliances for enterprises). I chose IPFILTER back in the day because it a) was less cumbersome than IPFW -- there was no PF then -- b) IPFW didn't support NAT (and it still doesn't except you need a userspace daemon), and c) back in the day IPFW rules were shell comands so your rules files were shell scripts. This is not a thing anymore but it was at the time.
As to why people use PF over anything else. I can offer an opionion. My opinion is people believe it's more secure than IPFILTER and IPFW because it comes from OpenBSD. IMO this is subjective.
I've done quite a bit to work out some of the niggling issues in IPFILTER. I've put IPFILTER work on pause for now as I work to import MIT KRB5 to replace Heimdal in base. There are a number of changes in yet to be completed git branches waiting for me to get back at them.
It's as solid as the other two packet filters in FreeBSD. All three are certainly easier to use than iptabes or netfilter in Linux.
Oh and BTW, BSD is dying. I've heard this at $JOB. I guess it must be.