Not only has it not been an issue...Tesla's share of plug-in sales has been increasing dramatically year over year.
Cadillac ELR, BMW i3, Chevy Volt, Chevy Bolt were all billed and hyped as "Tesla Killers" upon launch.
https://insideevs.com/monthly-plug-in-sales-scorecard/
To date, no other manufacture has announced any plans to procure enough battery supply to produce cars in comparable volume to Tesla.
To date. But more and more manufacturers are biting at their heels for the chance. With better and better cars.
that aren't selling nearly as well...
Perhaps, but people have been saying exactly this about Tesla for the last ten years now. Hasn’t been an issue for them yet, despite much improvement in the EV offerings from other manufacturers.