Sounds like it's still good to me. I would tension it up until it feels firm, DO NOT overstretch it! Everybody I've talked to recommends like 1/2 to full turn of the tension bolt. But this obviously varies since this saddle you've got seems like its seen some neglect. I can see some flex on mine when I push down on it. I'd also get some Proofide and get that leather conditioned again. Brooks saddles are incredibly durable and last for many years. I wouldn't write it off until the tension bolt has nothing left to offer and the leather has gone limp.