110M year-old nodosaur is the best-preserved fossil of its kind (2017)

I don't think it works quite like that. A DNA molecule (some people prefer to call it a "strand") has billions of nucleotides (or "DNA letters"). There is one DNA molecule per chromosome, and a few dozens chromosomes in every single cell. For all practical purposes, the same chromosome in various cells has the exact same copy of the DNA molecule. After the organism's death, each DNA molecule will probably break down in a matter of days, at most. From this point of view a half-life of 521 years is an astronomic number, and totally false. After a few years there will be exactly zero intact DNA strands. But that won't make a difference anyway, as we are not able to read a full strand in one read anyway.

Instead we read fragments and stitch them together. Like you see a "GATACA" and then a "TACACCG" and hypothesize that they were both parts of the longer string "GATACACCG". The longer the intersection string, the most likely the stitching is correct. In the mammoth case, they had fragments longer than 60 letters. Stitching together all these fragments, they were able to produce the full string (of billions of letters) 17 independent times. That gives them a high level of confidence that what they did is correct.

In the case of dinosaurs. Will we ever be able to read fragments of tens of letters and stitch them together? Maybe, maybe not. If the DNA breaks down so much that you won't be able to see even pairs of letters, then we are totally out of luck. If we see pairs of letters, we probably see triplets, quadruplets, etc. Can we stitch together a long string out of quadruplets? Most likely not, but if we have 10-letter strings, probably yes.

Are we going to have a contamination problem? If most of the DNA fragments come from bacteria rather than the original dinosaur, will we be doomed? Most likely we'll be able to come up algorithms that will eliminate bacterial DNA genes out of the reconstituted genome. We are quite smart.

Is Jurassic Park in the near future. Not before we are able to revive mammoths. And a mammoth is difficult to create for the simple reason that it takes a very long time to gestate a baby elephant. If each iteration in your experiment takes one year, you are in danger of shipwrecking your career.

Can we overcome these challenges? You bet. Anytime soon? My guess is that not during my lifetime, at least. But then Bill Gates states that we overestimate what we can do in one week but underestimate what we can do in a decade. So, you never know.