If the A-100s are markedly superior and worth the extra money I am sure they will buy plenty of them, they are not totally different aircraft... I would expect quite a few components of the A-100 could be added to the A-50s to upgrade them over time.
Remember these are not just radar planes.... that would make them AEW... these are AWACS, so they are the equivalent of a radar plus a command system... in the olden days that would equate to a radar and a control van to direct the defence via sending commands to aircraft and SAM units on the ground.
The Su-57 will be a more capable fighter in some aspects than an Su-35, but in other aspects it wont... its main feature is stealth, which it would lose when carrying a full payload, and of course it will be rather more expensive than any non stealthy equivalent... Russia was never planning to replace its entire fighter aircraft fleet with stealthy aircraft... and I suspect the ratio of stealthy and non stealthy will depend on a lot of factors... including what the west does... but they have the Mig-35, the Su-35, and the various Su-30s as fighters and the MiG-31 as an interceptor and the Su-34 as a strike fighter, so they are not desperate to get hundreds into service quickly... they can take their time and get it right.
Russia is a very big country and they could use a lot of AWACS and fighter aircraft, but they don't want to bankrupt themselves trying to match the US or NATO, or to be cool on the internet.