ERA is gigantically influenced by things that have nothing to do with a pitcher's skills
inherited runners, defense, official scorekeeping, park factors, etc.
using ERA to compare pitchers is a 1980s exercise.
FIP, xFIP and SIERA are INFINITELY more useful.
literally zero evaluators today look to ERA as anything meaningfully telling of a pitcher's skills or projecting success. literally zero.
get a clue
1. You don't get charged for inherited runners on your ERA. You only get charged for base runners who score that you put on base by hit or free pass.
2. The overall defense of each team is so close in the majors, that this isn't a big factor when differentiating between pitchers. You're only talking about teams that make more web gems, which is only a small difference. If your team makes more errors, you don't get charged for those runs and if someone makes an error preventing a 3rd out, you don't get charged for any of the subsequent runs that inning no matter what.
3. Official scorekeepers are equal opportunity offenders, giving hits where it should have been scored an error. They don't favor any one pitcher. You don't seem understand that it's not the ERA number alone, but how it relates to everyone else's ERA. In this light, the scorekeeper decisions are irrelevant.
4. If you look at the MLB Park Factors, Coors Field is the only true outlier at 1.17 of average. Most of the parks are .95 to 1.05 or closer. When you factor in the fact that everyone plays 1/2 of their games away, the effect to ERA is no more than -2% to +3% either way. This means that an average ERA of 3.0 can range from 2.94 to 3.09. A statistical non-factor.
The coaches I know use ERA and it's the #1 reported stat for pitchers at all levels of baseball.
SIERA is an
ERA indicator, just adjustments to ERA. FIP ignores balls hit into the field of play and only focuses on times when the hitter doesn't hit the ball, which is stupid and crap. It ignores the reality that good pitchers can force batters to not hit the ball so hard, others cannot.
Also, the use of FIP and SIERA is not as widespread as you say and is terribly flawed.
In any event, we're talking about
runs allowed by a pitcher, and not strikeouts per game like was mentioned earlier as a great metric for a pitcher. Kind of sad that you don't get that.