Subsonic because that makes the airframe cheaper and dramatically simplifies the computer programming for the flight control system.
Given unspoken purpose of the bomber is penetrating IADS and laying smack down on China and Russia, speed might be a valuable thing.
Basically the longer you spend over a target area, the greater the area of intercept, regardless of stealth.
Especially if new radars and systems fielded by these countries are as good as some US analysts and US Naval staff have indicated.
Indeed at least one senior USMC general has questioned utility of this platform, especially in a conventional fight against a large power.
And USN seems sceptical of stealth in a high density AD environment. They also seem to prefer non-stealthy F/A-18E/F/G to stealthy F-35C which they seem to be trying to avoid (e.g. attempts to have F-35 production delayed and having already slashed F-35 buy from 480 to 360 and stating that might be even lower).
Old saying is don't put all your eggs in one basket and the USAF has certainly put all their eggs in the stealth basket.
While ground-based national IADS are on a relative decline, ship-based IADS seem to be on the rise, with more and more nations building (or trying to build) aircraft carriers and the required escorts
Ship based IADS are a bit restrictive though (they are located on the sea). Bit useless in terms of guarding Moscow or Beijing or Tehran. :P
Other than Chinese carriers, there's no real push for new carriers, especially with potential US enemies.
The Russians talk carriers but don't put their money where their mouth is – since announcing grand plans for carriers in 2005 there's been no movement in this area.
Russians are focused mainly on bringing older ships back to life and new builds focus on submarines and lighter ships ala medium frigates and corvettes. There's been no building of large Airwarfare Destroyers.
Indeed all building of new "carriers" (mainly LHDs some of which are F-35B/AV-8 capable) has been for American allies – Australia (virtually no defensive capability), Japan, South Korea and Turkey.
The new airwarfare destroyers are also mainly entering service with US (Arleigh Burkes) and her allies (Australia, Japan, UK, Germany, Netherlands, Spain, South Korea etc etc).
As stated the Russians talk but haven't committed to anything.
The Chinese are investing in air warfare destroyers with 8 in service, 1 in sea trials and 6 more being built with more planned (Types 051C, 052C, 052D – all with displacement at 7,000 or more tons).
The only other major navy that is attempting to grow (but is being stymied by usual corruption and bureaucratic incompetence) is the Indian one.
They have 1 new carrier in service (ex Soviet Kiev class ship) and have 2 new builds on order. These replace an existing Harrier equipped carrier (old Centaur class INS Viraat).
They also have 3 AESA radar equipped large destroyers on order. None of existing destroyers are of the air defence design.
Most other countries continue to buy small frigates and corvettes with their "teeth" being submarines (usually Kilo or German 209/214 variants).
So I'm not seeing a growth in naval IADS in potential OPFOR navies. As stated the exception is China.
Stealth still has a purpose, even if the opponent doesn't have an IADS.
I don't see how stealth is useful in a Libyan, Afghanistan, Iraq (post invasion) or Mali type situation (most common type of operation performed by US military).
Also external bombloads decrease performance considerably – more drag = more fuel consumption = less range.
I actually think that's one of the big problems of the F-35. It's stealth ordnance load is not that great.
If the US is reorientating towards conventional war against major powers and especially in Asian region, then longer range with larger loads seem more logical than a tactical fighter that seems to be designed for Western Europe circa 1986 or Iraq circa 1991.