I vehemently opposed
Vehemence makes others stop listening and, at the same time, it limits our perception of the problems and the solutions.
To my experience, we become vehement at defending or opposing ideas when we don't fully understand what's going on, when we are missing something but we don't reach to see it.
Although shared entities sound harmless to me, I opposed the
idea of having shared repositories
MS architectures is anything but a trivial thing and splitting the persistence is among the hardest thing to do. This is something easier to say than done and thus why so many "gurus" write about "how things should be" but none say exactly "how to do it". Even we think we have found a good article, we have to recall that we don't know anything about the real context that led the author to make things that way. Or we do but we don't share the same problematics.
Making decisions upon someone else opinion only leads to mediocre decision-making, so don't be dogmatic. Be pragmatic. Put everything you know about the subject into the context of the project and pick those things that help you out to make things work. Start with the simplest solution possible because it will become complex eventually.
It violates S of SOLID. A microservice should only see & operate on data it owns.
One data source per MS is ideal but it's not a rule written in stone. It's not like Martin Fowler will appear and eat our soul if we don't do it that way. We'll segregate data sources when we find a good reason for it.
MS architectures have been promoted by companies with large monolithic systems which maintenance became so hard and expensive that made them less competitive in the market. These companies needed a way to articulate different SDLC so they could reduce the time-to-market, parallelize developments, gain the capacity to deploy new features and take'em off any time, etc. At some point they found that a single database was limiting their capacity to fulfil their goals, so they decided to split data sources too. Those were strategical decisions, not technical. Any architectural decision comes imposed by a business need which at the same time comes imposed by business strategies.
Developers under pressure would directly user these repositories in other services to modify data owned by other microservices.
Not necessarily, we can isolate repositories in several libraries and choose only those we deem necessary for each service.
In turn, we can make services connect to the data source with different profiles, each of which with different grants, roles or constraints. Many of the database engines support RBAC. Take advantage of it.
It leads to duplicate code and possibly missed validations
It leads to code-reuse and that's good. The libraries don't have to provide us with concrete data models if they were "abstracted" properly. Sometimes, the only we need is an interface to implement or an abstract class to adapt. The code-reuse in repositories is not the data model, is the logic around persistence. Regarding the missing code, if that code worth reuse, it will be encapsulated in the libraries alongside with the repositories.
It could lead to concurrency & data issues at scale.
Or not. Premature optimization is the seed of evil. Don't oversize the system until you have reasons for it.
The concurrency could be problematic if several services overload the data source so that the rest of the system suffers the consequences. That's a good argument for us to segregate the data source, but we have to wait until this becomes true. As for the race-condition over the data, it also happens in high decoupled MS architectures where it's even harder to deal with.
Are my concerns wrong?
You are theoretically right, however, you are not being practical enough. You are not paid to implement state-of-the-art architectures, you are paid to solve problems with whatever you have at hand.
did I miss any possible negative impacts (present/ future)?
Yes, but they only matter if they matter.
Shared libs mean that in order to take advantage of code-reuse, other services must be implemented with the same programming language. That's quite contrary to the MS philosophy1. But it's not necessarily bad if all your developers only know that programming language.
Code-reuse also couples SDLCs since changes in the common source code might penalize the delivery strategy, hence the time-to-market. But it helps to find what services are tightly related to each other. A signal that they could become a single service in the future.
Reality vs perception
Reality is all about perception. You can provide the "perception" of having different data sources when there's only one "real". Make it real when there's a real reason to make it.
1 - MS architecture is all about freedom of implementation and resilience