I can't speak from everyone's perspective, but the way I see things, the biggest problem with an Iranian backed Syria and a subsequent Shia block under Iranian influence (with backing from Russia which can't be ignored) is that makes Iran very strong in a geopolitical sense. Combine this with the lifting of sanctions and influx of money from the west because of the nuclear deal and Iran becomes even more powerful.

Until the younger generation of more secular Iranians can supplant the current theocratic regime, such a massive increase in power and influence is bad for Israel. In fact, it is bad for the entire Middle East. Not only will Iran be emboldened to continually threaten Israel out in the open (fearing no reprisal from the impotent west... and a threat which any sane person should take at face value), but conflict between Sunni and Shiite will deepen. Now that Iran has numbers behind them, they may feel a lot more at ease in challenging Saudi Arabia (who I think most consider as the de facto "leader" of Sunni nations in the Middle East).

From Israel's perspective, a Syria that is more friendly to Iran will allow more trafficking of weapons to Hezbollah in Lebanon. Not only that, but Syria could be used to traffic weapons to Hamas in the West Bank, as Iran is known to bridge the Sunni/Shia divide by supporting Hamas against Israel. So I'm not sure a direct attack from Syria is so much the problem as is the easy flow of weapons to those who have an expressed agenda of attacking Israel.

Also with regard to Haaretz, it is known to be very left leaning, so I would not be surprised if the "shared border" remark was just some colorful language to paint a right wing politician as a fear mongering war hawk.