Several reasons why:
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The amount of balance/content updates will probably slow down considerably - a lot of people are getting moved off the HOTS team to other projects. It doesn’t help that the announcement from the game director said “we’re still committed to regular hero reworks, themed events, and even new heroes” (emphasis mine) as if new heroes was something to be unexpected after the announcement.
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While I don’t watch any e-sports/streamers either, a lot of people do, and I’d wager that’s where a large part of the playerbase comes from - seeing people stream it. Killing the e-sports means people won’t stream it anymore, and that means attracting less players to the game. Playerbase is really important in MOBAs where you need 9 other people of comparable skill every match, so if numbers start dropping, that can start having a really bad cycle effect (it doesn’t help the last patch made the QM queue times way longer).
I’m thinking there might still be a decent amount of content coming out in the early months of 2019 since this decision sounds like it was made by the higher-ups as a complete surprise to the HOTS team, so there’s stuff in the pipeline that’s pretty close to done, but I’m thinking this game will be pretty dead come six months.