It seems they've got about 90,000,000 subscribers, so 500,000 is only half a percent drop. As the article projects, it'll still be many years before they're disrupted out of business.
The premise of the article is not that a 500,000 drop is significant by itself. It is that ESPN has huge contractual costs to teams/leagues to carry sports over the next several years that cost enough that they need (for example) 85,000,000 subscribers minimum in order to pay their overhead costs.
So it is not a 500,000 loss against 90,000,000 total, it is a 500,000 loss against a buffer of 5,000,000 [1] extra beyond the minimum subscriber point they require to bring in enough money to pay their obligations. 500k against 5M is a 10% reduction in their buffer zone in one month.
