It really depends on the features and what's really important to you.
If your existing Macbook is already snappy, and you can do whatever you want on it, there's little reason to upgrade, even if it's a few years old. Buying new updates frequently, you're basically paying another 100% of the purchase price, but you're only getting 5 or 10% improvement.
I got the Macbook Pro Retina, everything is snappy, PCI-E SSD and plenty of RAM. It just doesn't make sense to get another one for at least a few more years unless it dies on me or it starts crawling, which I'll doubt happen anytime soon. I think it's about 2 years old now (got it when they first introduced PCI-E SSD in it), I don't even bother to look at any newer updates. It fits my needs. When 700MB read/write is considered slow, then I'll start to worry.
For the Apple Watch, the main concern is probably battery and thickness. I see the first gen Apple Watch is like the first gen iPhone. If you look at their shape and thickness profile, they are very similar. After a few generations or battery and tech innovations, it'll be 1/3 the size. Maybe by then, the battery will last 3-4 days, depending on use, and up to a week if on reserve mode. Until then, you're going to have to do daily charging like the phones.
In a few years, it'll be past memories, and it'll get better and thinner. You won't be able to use any of your collection of bands from your first Apple Watch on newer generations, as everything is thinner and downsized. In 10 or 20 years, people will look back at the Apple Watch like they do with the cassette tape or CD players. Just enjoy what is being offered now.
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Now if you want to feel better about how much money you've foolishly spent on upgrades, watch this video:
https://www.youtube.com/watch?v=qXSpfwYL6eE