Here's how this should work, in practice. In a given game a bowler is more important than a batsman. Duh, there's only 4 of them, vs 6 or 7 proper batsmen + tailend contribution which you might add as another 1 or 2 proper batsmen in value. This extends out to a series, in which even a given seamer ( Shoaib Akhtar types excluded ), is likely to remain fit for the duration of, so a bowler is more valuable in a given series than a batsman as well.
Once you get out to a course of a year though, it 's really touch and go between batsmen and bowlers though, due to the impact of bowler injuries. A seamer is probably about 50/50 to take a knock putting him out of commission for one or more Tests. For some bowlers this has a greater impact than others, but I'd hazard it could be 70-80% of matches that a bowler ends up taking part in during his career that he is available for. A Test team's "player of the year" is more likely to be a batsman, as compared to a MOTM or MOTS being more likely to be a bowler. And once you get over the course of a career, seamers simply will break down much quicker than batsmen, just due to the accumulation of injuries. Unless they take some freak knocks, better batsmen should really be able to go on for as long as their reflexes and visual processing allow them to, which for most ends up being mid 30s, but some are luckier/unluckier in this regard.
Spin bowlers tend to be something in between in this attritional factor, but generally I think a good length seamer career (7 years +) ends up being about equal in impact to a decadish long batsman career (assuming they play a typical amount of matches for a bowler / batsman respectively in those years). If you get more longevity than that, the value is higher for the seamers than for the batsmen, imo. McGrath/Anderson get a huge boost as the almost never miss a match sort, and combined with McGrath's obvious quality and consistency when he did play it's so, so hard to match McGrath in my opinion for the GOAT bowler conversation.
If Steve Smith plays maybe 20% more matches, without any further performance drop, I think I could compare him to McGrath in value. I don't think there are any other comparable modern batsmen when it comes to value to compare to McGrath as a bowler, even Lara or Tendulkar.