Commercial aerospace strategy is a (small) part of my job. There's a short answer, a long answer and a REALLY long answer to that question. There's also a book length answer, but something like that might be coming out later this year
Short answer is - Boeing doesn't know exactly what it's going to build in the future. At least, not yet. But it's working on lots of things. But they probably won't be here until about 2035.
Long answer is - Boeing plans to keep building 777, 737 and 787 derivatives for at least the next one to two decades, and probably longer. Based on program accounting quantities and assuming full rate production can be indifinitely maintained, the 737 MAX will be in production for at least another 21 years, the 787 another 14 years and the 777/777X about 7 or 8 years.
Officially, new Boeing clean sheet designs are waiting on a new generation of engine technology offering 10-20% reductions in fuel burn to materialise. This is not expected for 5-8 years. Boeing has completed market studies for the NMA (think a modernised 757) and the NSA (787 technology shrunk down to 737 size), but is not willing to go to market without better engines on the wings.
A summary of the REALLY long anwer is -
1) A bunch of financial considerations.
Making a new aircraft is hugely expensive - the 787 cost somewhere between $28.5 and $32 billion to develop and get into production, and then Boeing has incurred another ~$30 billion to date in production, tooling and other program costs. Boeing needs huge financial resources to fund a new aircraft program.
Boeing is loss making. The company has lost just short of $22 billion on an operating basis since 2019. The 737 MAX debacle means the company will probably be loss making again in 2024.
Plus, Boeing has a bunch of other financial commitments at the moment. It's committed to paying down $15 billion of the debt it took on to survive COVID-19 by the end of 2025. Boeing also needs to pay a bunch of compensation to airlines/leasing companies for 737 MAX & 787 groundings and delays, along with paying for the airframe rectification work. In 4Q 2023 alone, this totalled $2.7 billion.
Basically Boeing isn't going to market with a new aircraft until it is in a better financial situation (and gets its equity and return on investment out of the toilet). Bear in mind, Boeing is a company with $16 billion in liquidity available right now.
2) Production and engineering.
Boeing can't figure out how to build a new aircraft cheaply enough for its own liking, particularly not aircraft with high proportions of carbon composite. The NMA design was supposedly close to going to market, twice. But Boeing pulled the plug both times because they couldn't find a way to manufacture the aircraft at the unit cost they wanted. Airbus is having a similar problem with the A220, a design that it acquired/took over from Bombardier. It still hasn't managed to get the aircraft to unit break-even, despite doubling the rate of production (to ~6 per month).
Boeing is also under pressure to onshore its engineering resources for any new aircraft. But it simply can't find enough trained engineers in the US at the moment to deal with it's current engineering demands, let alone developing a new aircraft. Boeing also doesn't really hire foreign workers in the US (H-1B visas) - less than 100 foreign workers are employed at Boeing Commerical Airplanes, out of a workforce of more than 35,000.
3) Uncertainty about future propulsion for commerical aircraft.
Generally speaking, an airline is only interested in a new aircraft if it offers at least a double digit reduction in fuel burn compared to the aircraft it's replacing. So, without any new engine designs available in the short term, OEMs are stuck offering refinements of their existing airframes.
The airline industry has also committed to cutting its emissions by 50% by 2050. 96% of industry emissions come out of the tailpipe of a turbofan or turboprop. So, the industry needs biofuels/SAF (which offer 60-80% lifecycle emissions reductions, but are only available in tiny amounts at the moment). The sector is also really interested in low/zero emission fuel sources and new engine technology.
There's a drive at the moment for hybrid-electric systems in the 2MW range for 50-100 seat turboprop replacements, with ATR and Embraer likely to put new aircraft into service with this technology around 2030.
Bigger aircraft are more difficult. Unless battery technology becomes an order of magnitude more efficient in terms of both weight and energy density, liquid fuels are really the only game in town for flights longer than 1000km. It's hard to find a fast charging point when you're crossing the Pacific.
So, the big engine makers (GE, Safran, Pratt & Whitney and Rolls-Royce) aren't offering new engines at the moment and they're all looking at alternative fuels for the future - hydrogen is popular, but there's also research into methane and ammonia. Which means that there's massive uncertainty at the moment about what sort of engines are going to end up powering the next few generations of aircraft.
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