EJets or A319s can do high utilisation and won't hurt the maintenance too much, but the lower capital cost will also help to ensure that if they have to sit on the ground for extended periods that it doesn't hurt. Having a Max on the ground awaiting spares is a killer, and doubly so if you're paying another airline to fly your schedule for you.
Not sure where you are getting the goss on the EJets being good for spare availability and reliability on high utilization. Maybe if you buy a few more for use as spare parts as well. Alliance had only recently planned to 'part out' 12 or so of it's E190s, reducing that to 7 due to aircraft shortages of late. Says something if until recently it was better selling your fleet as spares rather than ongoing assets.
There's a big difference in the advancement of the 100-200 seat jet market in terms of technology compared to say the 30-70 seat turboprop market, where the new machines are not much different to the old. A Max will deliver significant operating advantage over an E190 if it can be filled, probably even at 70% load given the right pricing. You just have to keep it moving around to make use of that advantage. The same as a Q400 can be operated cheaper than an ATR-72 by utilizing its positives, especially as either ages.