Replace it with a CON/WIS penalty until reset to represent mourning/weakening. Currently, familiars are only used as pocket spell casting items, or more commonly as locked door openers. The XP penalty at higher levels are simply prohibitive on an OOC level. It doesn't promote "mourning" the death of one's familiar nor associated RP.
It would be cool to lose one's familiar or RP one's familiar in dangerous situations once and a while, or even have wizard familiar competitions! It's just never going to happen with the current rules on familiar death, especially if you consider PVP situation. A lonesome familiar is a perfectly legit target to take down, and even warrants /c hostile all on behalf of the owner in most cases.
I like this, my suggestion would be that the penalty be to int and cha so it would have a more of an effect on the spell casting ability of the Mage/Sorcerer.
I have and have had plenty of fun with familiars, even using a silent raven to lead an unknown adventurer deep into the Badlands to escort my (now retired) sorcerer out of there.
The XP loss is fine I think, these familiars aren't disposable summonings, and if we followed D&D, losing a familiar means it can't be replaced for a year... Protect, nurture, play with them... But if you want to have familiar wars you may as well just use the pseudo tools provided to command summonings that can be bound for 24 hours than risk your sacred companion.
Replace it with spell failure imo
Spell failure until next reset is a fine idea. 5% by level.
If resets is too much of an arbitrary measure, we could use a the day counter employed for the waw favor system.
Have the wizard be stunned if their familiar dies, no save, to further empathize the psychic backlash. Perhaps also knocked down.
This is not necessarily to replace the experience loss. I just think it'd be a nifty addition.
Maybe familiars should be unsummonable for a week or more if they die - to put on emphasis that they aren't 'disposable' and greater care is needed with them.
Original d&d gives both xp penalty and inability to acquire a new familiar for a year and a day.
Maybe introduce a dialogue option for familiars where they will be automatically dismissed if attacked (combat is fast sometimes, if a player attacks your familiar and doesn't kill it right away you'd want to do this as quick as possible, which might be difficult while possessing it.)