The Sprint Backlog is the set of Product Backlog items selected for the Sprint, plus a plan for delivering the product Increment and realizing the Sprint Goal1. The Sprint Backlog is a forecast by the Developers about what functionality will be in the next Increment and the work needed to deliver that functionality into a
"Done" Increment2. The Sprint Backlog is a plan with enough detail that changes in progress can be understood in the Daily Scrum2. The Developers modify the Sprint Backlog throughout the Sprint, and the Sprint Backlog emerges during the Sprint. This emergence occurs as the Developers work through the plan and learn more about the work needed to achieve the Sprint Goal2. The Sprint Backlog is not a commitment to deliver a fixed scope of work, but rather a forecast of what the Developers believe they can do in the Sprint3. Therefore, the Sprint Backlog does not need to be fully defined during the Sprint Planning, but only enough so the Developers can create a realistic forecast of what they can do during the Sprint. References: 1:
Sprint Backlog, 2: The Scrum Guide, 3: Commitment vs. Forecast

Explanation
= As a Product Owner, you are accountable for maximizing the value of the product and the work of the Developers. To do this, you need to have a clear vision of the product and its target users, as well as a validated understanding of the market opportunities and risks. You also need to collaborate with stakeholders and customers to align their expectations and feedback with the product goals and strategy.
In this scenario, you face a dilemma between pursuing a new market segment or satisfying the existing customers. Both options have potential value, but also uncertainty and trade-offs. The best way to proceed is to devise an experiment that will help you to test your assumptions and learn more about the new market and its potential. This could be a small-scale release, a prototype, a survey, or any other method that can provide you with empirical evidence and feedback. By doing this, you can reduce the risk of investing in a feature that may not deliver the expected value, and also gain insights that can help you to refine your product vision and backlog.
This approach is consistent with the principles of agile product management, which emphasize delivering value early and often, validating hypotheses with data, and adapting to changing customer needs and market conditions. It also demonstrates your ability to apply the Scrum values of openness, courage, and respect, as you are willing to explore new possibilities, challenge your own opinions, and involve your stakeholders and customers in the decision-making process. References := Scrum Guide, Managing Products with Agility, Evidence-Based Management

According to the Professional Scrum Product Owner II certification guide1, the Product Owner is accountable for maximizing the value of the product resulting from the work of the Scrum Team. This means that the Product Owner should have a clear vision of the product, understand the needs and desires of the customers and stakeholders, and prioritize the Product Backlog items based on their value and urgency. The Product Owner should also use evidence-based management to measure the value delivered by the product and make informed decisions about the product strategy and direction.
In this question, Product A has a high current value, which means that it is generating a lot of revenue and satisfying the existing customers. However, it also has a low unrealized value, which means that it has little room for improvement or innovation, and may face competition or obsolescence in the future. Product B has a low current value, which means that it is not generating much revenue or satisfying many customers.
However, it also has a high unrealized value, which means that it has a lot of potential for improvement or innovation, and may capture new markets or opportunities in the future.
Taking a long-term view, the Product Owner should weight the investment toward Product B, since it has more potential to deliver value in the future. This does not mean that the Product Owner should neglect Product A, but rather balance the investment between the two products based on the expected return on investment and the risk involved. Investing equally in both products may not be optimal, as it may result in underinvesting in Product B and overinvesting in Product A. Weighting the investment toward Product A may not be wise, as it may result in missing out on the opportunities offered by Product B and losing the competitive edge in the market.
1: Professional Scrum Product Owner II Certification | Scrum.org