Looking at https://moneroblocks.info/stats, the current growth rate looks to be about 400 MB per month, average over the past last half year. This is about 5 GB per year. This means reaching 100 GB (+30 GB) in 6 years, and 200 GB (+130 GB) in 26 years. This is assuming no increase nor decrease in usage, nor in software improvements (transactions are likely to get 25% smaller in the next fork).
Monero will support pruning in the coming release. a 70 GB database prunes to about 25 GB. The effect is smaller on current bulletproof transactions than previous Borromean proof based transactions though. I'm not quite clear what the practical growth will be for a pruned blockchain compared to 400 MB, but it should be significantly lower, increasing the times above.
To your last point, blocks do not get smaller with more usage. They will get larger, since they'll have to store more transactions.