«Hasn't been true»

Yes it's true. I said WEIGHT not SIZE. A distinction few understand. Check the kWU column at https://blockchain.info If a full node has built a 4Mweight/1MB block, it is full, because non-segwit txs have wasted the 4M weight quota and prevented the block from growing over 1MB. If segwit adoption stays low, blocks WON'T be able to grow much beyond 1MB.

In fact the problem is that the rate of adoption of segwit is too low compared to the rate of increased txs that Bitcoin needs to be able to support. Therefore blocks are going to continue to stay at or near the ~4M weight quota.

As to evidence for users moving from Bitcoin to alts because of full blocks:

The tx rate of ETH, LTC, and other alts started sharply increasing in March/April 2017, which is exactly when Bitcoin blocks reached capacity (~250k tx/day)