First, whatever was right and wrong, OP is unlikely to ever recover any appreciable part of the $5K he spent and he is unlikely to recover any of the AB ticket value which remains from AB. The only argument he has as to the $5K is that it is a post-insolvency commitment under EC 261/2004 because AB had an obligation to rebook. But, that claim (along with the EUR 600 for the presumable delay caused by the not noticed cancellation) will be an unsecured claim against AB. There is close to no chance that there will be funds left to pay the claim so OP should not count on anything. In fact, he should count on nothing.
2. While it is unfortunate that AB did not notify OP individually (perhaps he purchased through a TA, OTA, third-party vendor?) the fact is that AB's situation has been well known since mid-August and the fact that it was to cease long-haul operations was all over the media. That is not an excuse for AB, but given the situation, it would have been a good idea for OP to be much more proactive.
3. Again, not to berate OP, but he had a good option here. Same day tickets SFO-LAX start at $79 and are sometimes even less. There are also 50+ frequencies/day on multiple carriers. Before paying $5,000 on the verbal commitment of a staff member, it would seem that spending $79 and then fighting about that later, would have made sense. A better way to look at this would be to ask oneself why on earth AB would offer to reimburse $5K rather than FIM the ticket to another carrier if it had the funds to do so. Similarly, $5K seems to be an astronomical fare. At a minimum, there must have been vastly cheaper return tickets available, even if OP chose to abandon the return segment.
4. I suspect that all that is left for OP is to carefully read his travel insurance contract and, if it covers this situation, make a claim there. Depending on where and how he purchased his ticket, he may also be able to obtain a refund through a chargeback of the unused segment of his AB ticket.