Union incentive align mostly to more members not more salary or benefits. Similar how companies tend to go for more revenue unions benefit more from additional member and less from higher salaries
The two are not mutually exclusive. The incentive for seniority-based pay is that low-performers are dissatisfied with lesser career advancement, so they form a block within the union electorate and vote in leadership that flattens pay.
You are correct that unions are also incentivized to increase membership, that's why unions often oppose more efficient work practices. More efficiency reduces the number of personnel needed to perform the job. An example of this is the longshoremen's unions in the US. They have consistently rejected the automation that most other developed countries have long-since adoped [1]. The result? American ports are some of the least efficient in the world, despite also being quite expensive [2].
