Nikon's rumored (almost certain) new mirrorless systems will almost certainly have a sensor size and, more importantly, lens mount that is different from the CX mount system of the Nikon 1 series (of which the J5 is one).

Does it mean that I should not go for it? Why?

This means that going forward Nikon are almost certainly not going to make any new CX mount lenses (for the Nikon 1 series) and as no one else does, that system is now effectively dead in terms of future development and expansion.

As cameras they are, if I may put it politely, as good as they ever were. Which, less politely and IMO, is not good at all.

Only the Nikon AW1 had any real selling point going for it over the competition, which was it's water-proofing.

Going forward I'd advise you to invest in one of the other mirrorless platforms which look likely to continue (micro 4/3, Canon-M, Fuji-X, Sony E and FE). As for the likely success or otherwise of Nikon's "new" mirrorless system, I am skeptical as they made such a dog's dinner of the Nikon 1 and basically ignored what people were telling them, which bides ill for their chances in a very competitive market.

Does it mean that I won't get any replacement or support if it is broken under warranty?

Only Nikon know exactly how they'll handle that (and as usual they're telling people nothing). All I would suggest you depend on are the consumer rights in your country. I would hope that Nikon at least respect their own warranty but it's hard to see them providing replacement parts for a series of cameras they have discontinued. It's likely they would have some small stock of spares for a while, but I would not depend on that.

Just like smart phones, laptops, etc, I always go for older models for cheaper prices. The latest ones are not worth buying in my view - unless you are rich!

Good plan. Also consider older used models from dealers. These can have good value and be surprisingly similar to new models. Dealers also typically give a limited warranty with used gear and will also grade it, usually fairly.