To add to airboss, till you have 10/15 aircraft, focus solely on that aircraft type/family. Each new aircraft family is a significant maintenance cost, so until you have a steady steam of money flowing, don't expand out of that. Always do 10 year leases, and once you recieve the aircraft, extend them again for another 5 years so that the lease is even further reduced - small things can help a lot in the short/long run

Plus if we're talking about the mega airlines that have 5000+ aircraft, you need to maintain the old planes that are no longer in production in order to keep growing faster than your competition. If you get rid of the DC-9s, you need to replace them with a replacement aircraft type, thus loosing the growth potential of adding the new aircraft straight into new routes. Without a doubt the efficiency of the DC9s will be decreased and their overall profitability will be significantly hampered, but they're still producing more profit and growth than had you replaced them with say a 737, and by replacing them with the 737, missed out on actual growth by adding new routes. 

In an idea/realistic situation you'd have all the mega airlines operating MAXs/NEOs/350s and 787s, but realistically, they're going to be operating every aircraft type since the beginning of the game, because it is the only way you can maintain top 10 in any game world. 

Leasing a plane is nearly always more expensive than outright buying it. Leasing planes is good for when you start because the cost of having it within your fleet is minimal compared to actually buying it outright, especially with Widebodies that cost 200+ mil. I've personally never scrapped a plane. I try and keep my fleet as young as possible after having reached market saturation, and renewing it with either the same model or upgrading (if available) - say A330 with A350, 777 with 787 etc. But I've never scrapped an aircraft because selling it always makes more financial sense. 

Depends. If its profits your after, then 3 high seated 747s. But operating 3 weekly flights won't give you a good route reputation whereas operating 10 A320s will increase it significantly. Also depends on route length and cost of airport gate, and if you have various routes of 700 pax demand into it, then if you have various 10 A320s operating that route you're gonna use more gates (thus increasing prices). But if you whack 3 747s then you'll probably be good to go on one gate.