I'm sure the OP is long gone, but I'll add my two cents anyway (adjusted for inflation). Deck delamination isn't necessarily the kiss of death for a boat; I think it all depends on the boat and the expected use of the boat. I had an Oday 23 that I bought 20 years ago (actually still have it; anyone interested?); she had a soft spot on the foredeck right behind the forward vent. The surveyor told me that if all I'm going to do is daysail or overnight on Long Island Sound, that this condition wouldn't be a problem. He told me to rebed some hardware near the soft spot and I'll be fine. He turned out to be exactly right. For years, the soft spot didn't grow, and it was never a problem. Granted, I never went out in really bad weather, but I sailed in some rough waters over the years. No oil canning, no flexing, no nothin'. Eventually, the soft spot grew somewhat, but it's still not an issue for the use I made of the boat. There's just no way it's worth fixing the core on a boat that is worth $3000 on it's best day. Forget about the time it would take, forget about the cost of the materials; just finding and paying for an indoor enclosure to do the work knocks it out of the park. I know there are people who love this kind of project, and would enjoy the process. I would probably like to do it myself, but it's just not that important to me. Especially when it's not necessary. I've always had enough trouble keeping up with necessary maintenance tasks.
And are there 30 year old boats without some deck delamination? Of course there are. However, in my humble experience, deck delamination is more likely to occur on smaller, more value-priced boats. First off, those boats aren't usually engineered as Jackdaw points out with solid glass at attachment points. Second, these boats are usually bought by value-oriented and less-experienced buyers, so they may be less inclined to know about or care about rebedding deck fixtures or trying to address deck leaks. The result: more of these boats have deck delamination than maybe the general sailboat population. Counting roughly, I'm guessing that about half of the 22-25 foot boats I've been on have at least some spongy deck.