There needs to be a careful balance, and that is hard to strike on the first blow. I did a little mining, and I am still a rookie pilot by any stretch, and from what I saw the income potential of mining was very VERY high before. Especially if you did the meta thing and played with the trade boards and hopped around to sell at the best ports.
However from what I see now, Combat missions have one of the larger RoIs. Especially if you are stacking pirate wing missions like my friend and I did a week or so ago. Spend 1 hour killing 25-45 pirates, and getting 30 mil....vs Spending that hour hunting for, and mining a specific resource, and only getting 15 mil.
It is super easy to find and kill pirates, either with signal sources or just going to a res site and popping the pirates you know you can take. With little risk to yourself, and your costs are offset by the bounties you get from all the ships you kill.
Mining however is a much more arduous process, and while 'safer', you still needed to know what you are doing and what you are looking for in order to make the most money and even then it paid far less than combat wing-mission stacking in my anecdotal experience.
That said I agree mining needed a nerf, but the amount would be scale of how sharply the decline is. I have not dabbled in mining since the nerf, since I am currently out in the black in my DBX. I never really did any 'free mining', I always mined with missions in mind and specific resources I was supposed to collect and turn in, not flipping the commodity market.
So it depends on how the nerf manifests, if it is just a nerf to 'free mining', people stocking up on rare minerals and flooding a market with them, then of course it needed to be nerfed, What the hell is an agricultural station going to do with 3000t of void opals?
However so long as they leave mission-board mining alone, then what is the problem really? Let stations tell you and reward you for the minerals and materials they really need, stack those and use those to make your millions. In this regard I would love to see a buff for mining missions, to compensate for the abuse of free-mining, after all if a station is telling you they need a specific material, then of course you should be rewarded for bringing it. Just like free-bounties pay garbage compared to stacking pirate missions.