As I quoted earlier, Trump pointed out the fraud potential in doing so, especially given the little time remaining before ballots have to go out.
This isn't about preventing fraud. Mail-in is proven to be secure. Take the words as exactly what they are--limiting votes. He directly states he wants to limit votes. There's no need to try searching around for other meanings when his statement is so direct.
[1] https://www.brookings.edu/blog/fixgov/2020/06/02/low-rates-o...
Oregon has also been 100% vote-by-mail for two decades. Other states cannot replicate Oregon's experience and infrastructure—often from scratch—by early October.
What a specious argument.
In fact Trump's rhetoric will be especially motivating for pro-Trump fraudsters who will feel morally vindicated by the endless accusations of fraud by Democrats.