I think it's unfair and discouraging for newcomers that old users have billions in score on Stack Overflow.
That's... really a problem with the mindset of newcomers: They compare themselves with people that have had years and years of time to reach a certain 'goal', and want to reach something similar in a month, or 24 hours. That's not realistic. If you do want to compare yourself, compare yourself to your peers, so people that started around the same time as you, under similar circumstances.
If you really want to compare yourself with users that were active when a site just started and the 'easy' questions were lying around for anyone to ask, don't compare yourself with their score now, but with their score the way it was when their account was your age. You could do this 'for fun', but realize that you're making a comparison that's similar to comparing the financial stability of someone that's 25-years-old in the US now, with someone that was 25-years-old in the US in the 60s: for the current average 25-years-old buying a home is a lot less easy than for the one from the 60s.
I think it’s unfair because to some degree i [sic] think your score should reflect the amount of effort you’ve put into helping the community
That's not what reputation on these sites is for! From the help center page: Reputation is a rough measurement of how much the community trusts you; it is earned by convincing your peers that you know what you’re talking about. Reputation doesn't reflect effort, it reflects whether you know what you are talking about. So giving people less reputation just because there's no longer any 'effort' makes no sense: the reputation they get awarded, they get because their posts are still relevant to others, their solutions still work, they still know what they're talking about.
Even if reputation were to reflect "effort", and be given out for e.g. casting close-votes, casting delete votes, raising spam flags and other efforts that keep these sites clean and useful, in a healthy system a new user will never catch up with an older user if they're both putting in the same effort. It would quickly become unhealthy to try to do 10 years of effort in a month: That would mean resorting to things like robo-reviewing, and that's not a good effort that leads to a quality site.
In the end, we're talking about a bunch of pixels on a screen here. Comparing your arbitrary number to that of others is not the healthiest way to exist on the internet. Don't rely on a comparison to others to build your self-confidence or get a dopamine release. Instead, realize that whenever someone upvotes your posts as helpful, or whenever you do your part in keeping these sites clean and useful, you've done your job and can be satisfied, proud, or whatever else you need to feel. There's no need to change the reputation system for you to be able to feel that way.
It should instead incentive answering hard questions & bring more competition between new & old users which enables our field to reach new highs
You think that just because people will no longer earn reputation for a very, very useful post, they suddenly will have both the expertise and the motivation to just go answer 'hard' questions? That's not how any of this works...
These sites aren't here to 'enable our field to reach new hights'. They are here to build a library of knowledge, where people can find what they are looking for. You're free to add as many questions and answers to that library as you can provide (as long as you don't end up quality banned), but in the end, usefulness is determined by how many people find the information you add to the library, and can use it. Not by how 'hard' the question was to answer.